Alright, just finished the game after playing for 12 hours (mostly off-screen whilst doing other stuff). There really is very little reward. The ending is a simple scene with Mercy and a "You won!" text box. The cost-balance is all out off whack, most of the time the best thing to do is just hit rebirth and spam buffs together to get compounded effects. For some reason, these buff upgrades are super OP for the price, but getting to them is horrible. I mean, I just spent 12 hours playing so I can't complain, but for most people, stick to the first two girls, specially Eve and save yourself 11 hours.