D&D 5E Questions on flaming sphere and liquid tendrils

[The Crimson Binome]

2) Assuming I'm looking at the same thing (I have Water Whip on my reference sheet, but it's identical to what you have listed), it costs 2 Ki.

It's also worth mentioning that there is a typo in the book, and the ability actually requires an Action to use (instead of a Bonus Action). You can tell because all of the other effects that cause that much damage all require an Action to use, so this one would be inconsistent if it was the only Bonus Action ability, but they clarified it in the errata just so everyone knows for sure.