It doesn't pay to get all your girls to max XP, only the ones you plan to use in battle. You will find as you get higher level that you will need those books you are stockpiling just to get your harem to raise level.

I agree with Zarelion about playing the 10x pachinko. You can usually at least get your money back by selling the items you don't need. This will also make it easier and cheaper to get those high price equipment items you are saving for. I never bother with boosters.