While xp cost is set to 0 on both client and server, xp doesn't drain properly however I couldn't fly when I was out of experience (Or more accurately when the game thought I had no experience due to ATM7 having a client-side bug that registers you as having 0 experience until you gain some again after switching dimensions)

This is some sort of an issue out of my scope. However, there's a mod fixing this already. I think I'll implement the same patch in near future since it's MIT.