The Rasenshuriken?

"Well, Kakashi," said Jiraiya, giving him a resounding clap on the back, "Naruto's all yours now, just as I promised."

Naruto groaned, gingerly rubbing the black eye Sakura had given him. Compared to his two teammates standing next to him, he cut a rather sorry figure…

"You're not staying for the assessment, Lord Jiraiya?" asked Kakashi, feeling a bead of sweat begin to trickle down his back. "If Sasuke's progress is anything to go by, I doubt I'd last long enough against all three of them to get a proper measure of their abilities."

"Afraid not," replied Jiraiya, wincing slightly as he pressed a hand to his sternum. "The Akatsuki are on the move, so duty calls."

In truth, Jiraiya wasn't in any state to be fighting anyone, having spent much of the previous month in the hospital recovering from his injuries. To accelerate his pupil's training, he had loosened Naruto's Eightfold Trigrams Seal, confident the Child of Prophecy could handle more of the Nine-Tails' hatred.

Reality, however, proved far crueller than he had expected.

During training, as Naruto sprouted more and more tails in Demon Fox Mode, he was almost immediately overcome by the Nine-Tails' malice and came close to mauling Jiraiya to death. To make matters worse, the evil chakra had peeled off most of Naruto's skin, so he too had to be hospitalised for a while.

"Ah!" Sakura chirped. "Before you go, Jiraiya-sensei, could you please teach me a Jutsu?"

Jiraiya, who had already turned on his heel to leave, glanced at her over his shoulder, sneaking a peek at her cleavage as he did.

"Why should I?" said Jiraiya, feigning a lack of interest. "I've already gone out of my way to teach Naruto here, and I'm a very busy man, you know…"

"Please," said Sakura imploringly, bending forwards slightly to give him a better view of her low-cut neckline. "I'll do anything…"

Jiraiya stopped dead in his tracks.

"Anything, you say…?" he said slowly.

Wasn't this the sort of development that only happened in those raunchy novels he wrote? Did that sort of thing really happen in real life!? After all these years, was it finally spring for this old man? Had his popular season finally arrived!? If he could motorboat a pair as nice as Tsunade's, he would die without regrets!

"Jiraiya…" said Tsunade warningly.

She slammed her fist into her open palm with an audible smack, leaving no doubt about what would happen if her old friend tried to put the moves on her young disciple.

"How about an equivalent exchange?" Jiraiya hastily proposed. "You'll show me yours, and I'll show you mine."

"JIRAIYA!" roared Tsunade. "If you lay even a single finger on my disciple, I swear you'll never walk in a straight line again!"

"Phrasing!" said Sakura emphatically.

…

And a few minutes later, Sakura's wish finally came true.

No, not that one.

She no longer had to work long hours at the hospital, because Tsunade had just reassigned her to Team 7. Only this time, Naruto, Sakura, Sasuke and Kakashi would be forming an elite, rapid-response, anti-Akatsuki team— but not as students and sensei, but as equals. No longer were Sakura and Sasuke little Genin learning from a seemingly untouchable figure…

They were all grown up, now. Jōnin… except for Naruto, who was still a Genin.

Lol.

After the misunderstanding with Jiraiya was resolved, Tsunade left in a huff to attend to her pressing Hokage duties, while Naruto, Sakura, Sasuke, Kakashi and Jiraiya made their way to the Third Training Ground— the magical place where each of their shinobi careers had truly begun.

"I heard from Tsunade that you've improved the Rasengan, so let's go with that," said Jiraiya once they had arrived. "If you can show me an Elemental Rasengan, I'll gladly teach you any Jutsu I know."

Personally, Jiraiya was very sceptical of Sakura's story.

After all, she claimed to have added a Nature Transformation to the Rasengan— but apparently, she had done it only once, and with no witnesses to boot. The Rasengan already represented the pinnacle of chakra control outside of the body— the ultimate form of Shape Transformation. To top that with a Nature Transformation would demand parallel thinking skills the likes of which the world had never seen!

"Well, which nature would you like?" asked Sakura, planting her hands on her hips. "I can manage either Wind or Lightning."

"Why not fire?" Kakashi asked sceptically. "Didn't you write Fire Style: Rasengan in your mission report, after your fight against the Akatsuki?"

Since fire was Sakura's main nature and she only had two available slots in her Breast Seals, she'd loaded them up with wind and lightning chakra respectively, the next two natures she was best at. As the evolved Rasengan demanded more brainpower than she could muster without Shadow Clones, Sakura was only able to use it in conjunction with the pre-moulded chakra in either of her two Breast Seals to vastly simplify the process.

So, no more Fire Style: Rasenshuriken.

"Let's go with wind, then," said Jiraiya, glancing in Naruto's direction.

When it came to the Rasengan, there were three ways of performing the technique, though the method itself didn't affect the final result. Each approach, however, revealed one's level of mastery:

The Novice: Four-handed, using a Shadow Clone as a crutch.

The Journeyman: Two-handed: one hand doing the spinning and the compressing, and the other hand doing the containment.

The Master: One-handed.

The Grandmaster: No-handed. (Look, Ma!)

Currently, Sakura was able to do the normal Rasengan one-handed. If she were so inclined, she could always try her hand at manifesting one from anywhere on her body (which would have been a colossal waste of time).

But to perform a Nature Transformation, she did need both of her hands to be unoccupied.

"Okay, here goes nothing," Sakura muttered to herself. "Shadow Clone Jutsu!"

Poof!

A second Sakura appeared next to the original, but curiously, she just stayed where she stood instead of helping. The onlookers quickly turned their attention back to the real Sakura when it became apparent that the Shadow Clone wasn't part of the process.

Sakura extended her right hand in front of her and placed her left over her right. She concentrated, leading the wind chakra sealed within her chest down her arm and emitting it through her hand's Tenketsu. 

First was rotation.

Then was compression.

And lastly was containment!

In no time, a swirling ball of pale-yellow chakra coalesced in Sakura's palm. The nearly perfect sphere started emitting a high-pitched buzzing, as the wind chakra inside strained against the containing envelope of chakra outside. Sharp blades of wind whipped furiously around its equator like a shuriken's edges, as the surrounding air got drawn in by the rapid rotations.

Kakashi's and Jiraiya's eyes widened.

"Such chakra…" murmured Kakashi.

"Well, I'll be," Jiraiya chuckled softly. "She's actually done it."

…

Welcome back to the Learning with the Busty Kunoichi Sakura's Corner!

How much chakra is there in one Rasenshuriken, anyway?

Moulding Senjutsu chakra is 1.5 times more efficient than kneading regular chakra, considering the ratios of energies involved (1:1 and 1:1:1). Since spiritual and natural energies can be considered to be more or less unlimited, physical energy remains the only limiting factor when moulding chakra.

When Naruto gathers natural energy to initiate Sage Mode, he is limited to using only his own energies— which would add up to his natural 4 KK cap, should he infuse them into chakra. In exchange for this limitation*, he gains a 1.5-times boost in chakra generation efficiency, along with a considerable amplification to his strength; speed; senses; reaction time; and to Ninjutsu, Genjutsu, and Taijutsu potency.

*(That limitation really only applies to Naruto, as no one else possesses a portable tank of infinite chakra like his. Moreover, it becomes irrelevant not even a few chapters later. Kurama can forcefully supply Naruto with chakra even while he's in Sage Mode, and later still, when they become friends, Kurama will even gather natural energy on his behalf.).

At the start of the series, base Naruto can produce three Rasenshuriken in a row, thanks to the Nine-Tails' chakra leaking through the seal aiding with his energy regeneration rate. However, in Sage Mode— restricted by the aforementioned limitation— he can only form two, since Senjutsu chakra doesn't regenerate on its own.

Using maths, we can deduce that one Rasenshuriken costs between around 1.3 KK to 3 KK. The lower bound's value is a bit iffy, since Naruto's chakra regenerates extremely fast, allowing him to squeeze off a third shot in quick succession even if the total cost should exceed his theoretical maximum.

So, let's just round it up nicely and say it costs two entire Kakashis to fire one Rasenshuriken!

That's enough to use eight Chidori! Wow! What a bargain!