I've been already investing for almost a year in 3-4 different sites, in a month you can expect from 2% to 5% on average, having higher profit than that or a loss is not likely. This can only be applied to established casinos with big enough bankrolls to be stable. Also I started recently investing in YOLOdice, they allow up to 10x leverage on investments, which I'm using and currently finding it pretty good for relatively small investments.
People should really invest into the well-know and trusted dice sites anyway. For a few reasons. First, there is less chances that the casino will run away with your investment, but don't ever think that a well-know and trusted dice site will not run away with your money. Second, there are more people gambling in these sites hence more chances of profit.