I've only reached tier 8 with my Mogami, I can't say anything about tier 9/10 costs, but I like to play for fun, and not to stress because if I use a tier 8 or above ship I'd be losing credit to buy other ships I want to, I haven't lost money until now (maybe because I havent noticed losing money because of the premium account since now) but I just got premium for faster exp earning, but now, starting to look at the credits without premium accont I'm really discouraged to go above level 6 if the game economy doesn't change.