Mana values for various things are all over the code, mostly in mana-related items, blocks, and block entities. You can find some of them in the various (generated) recipes as well. In general, there is no central place, you will have to go hunting for everything.
The recipes can be changed by defining data packs, no code modifications needed. Mana consumption of things not defined in recipes are a lot more tricky to modify without outright recompiling Botania, though. I remember someone on the Discord mentioning an add-on project for making everything in Botania configurable for mod packs, but I can't remember who that was or what the name of that project was.