One of the game's designers had this to say on the Apocalypse World forums during the playtest:
You call up a horrible conflagration to burn your foes, but you want to send it away when you're done, so you give it your blood and take some damage (or sacrifice something valuable or some other method of harm) and you can send it back to the elemental mess it came from. If you can't pay the price, the leash comes off...
When Leafwillow chooses to have to pay nature's price, that means that for its task to be finished, nature will have demands to make of her, so it doesn't go out of control.
Alternately, if Leafwillow chooses not to be in control, that means that in order to do what needs to be done she exposes the people involved to danger or risk because the elements are out of control.
The exact details of what's going to happen, the risks to take or the price to pay, are not known to her in advance. Elemental Mastery isn't supposed to feel predictable, like the known consequences that happen when Wizzrobe or Clericsdottir Casts a Spell and gets a 7-9. Leafwillow is poking nature, and nature is a lot bigger than her.
As the GM, you have carte blanche with the price to be paid, but don't be worried. You didn't kill your players the first time they rolled a 6, right? Even though you could do whatever you wanted? You made a move that followed, and it's the same here. After the dice hit the table and Leafwillow is weighing her options, you can be more open about the price to pay. (But maybe not; maybe willingness to trust in Nature is itself part of the price.)
Some possible paths forward if you're stuck:
Turn their move back on them. Ask Leafwillow what she offers in trade. It's possible her answer will be enough inspiration, but maybe it isn't or she punts it back to you (with like a "what do you ask of me, great fire" sort of thing) which is fair, you run the universe.
Exploit your prep. It isn't the element's price in specific, it's nature's price. Leafwillow summoned the elements in a location. Did you prep that location? What does the location want?
Blood's good; you need blood to live. Arguably this is the canonical example, see quote above. It is at the least heavily hinted by other playbook text, like the bonds, that some part of the Druid's rites involve blood. So there's an easy answer for a price to pay.