% return depends on customers. I cant predict what they gonna do. Just-Dice i trust most but they use clam coin now, i not like it so betking.io and poloniex i trust most, but poloniex rates are going down atm. Just invest a amount you feel comfortable with and have patients.
I would probably invest only an amount that i feel comfortable investing,, but i just wanted to know more about those services of investing in bankrolls, because i heard a lot about them but never tried one nor i heard someone talking about their profits on this forum, people just say investing in casino bankrolls is profitable, and that's all.