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.
In order to figure out roughly how well your investment should perform over a year, take the average daily amount that is gambled at MP, multiply this by 1.825, this is the amount of returns the bankroll should earn over a year with a 0.5% house edge. You can then divide this amount by the current bankroll amount to get an estimate as to how much you are expected to earn on your bankroll investment on an annual percentage basis.
The above assumes no betting growth and no bankroll growth with is unusual for a new site like MP.
The above assumes that bankroll earnings will be withdrawn as they are earned (unlikely), assumes no betting growth (unlikely) and no bankroll growth (also unlikely), however it should give you a rough estimate as to what you could earn. There will also be variance which means that earnings will not be steady. The above assumes no betting growth and no bankroll growth with is unusual for a new site like MP.