what's the time interval before it resets to x1 end cost? assuming that it resets between battles, i'd calculate/guesstimate the avg number of phases you expect the character to use his power per battle and take the avg of that and compare it to the xN end cost limitation as use that as a guide.
e.g.
say you expect to use the power 5 times per battle
(1+2+3+4+5)/5 = 3
so i'd just take a custom modifier at the same minus as x3 end
if it turns out differntly than expect, your gm and you can adjust it later