Let's say I can invest 0.20BTC for 6 months, is there an average on what I would be earning or even a potential, thanks. 
No one can say how much you can earn in that timeframe as your profit will depend on how much the house wins against the players, if the house loses ofcourse you will lose too so there is no guarantee
There should be a semi predictable amount of return if you know the daily betting volume. I am not sure what it is, but Quickseller put up a formula a few pages ago that shows you how to calculate it.
No its not. It's called variance and happens in the short term. If you know daily betting volume and the houses edge, you can easily calculate what your expected profit should be. In the long run you will be right on, in the short run, you'll show variance. It's called math and it's absolute.