Oh, it'd have to be much less than 4000 NG; that's way too much for one guaranteed 5*+ unit in a 10+1. I would say 2500 NG is much more fair considering almost all but the last two step-up gachas have offered an 11 pull for 300-500NG.
But yes, buying FGs to spin is an utter waste; if you're willing to spend money on the game, then you get the most bang for the buck in using FGs for stamina refreshes.