But seems like mathematically it's unprofitable, because when a player loses - I as investor get only 50% of my profit taking into account the casino's fee, but if the player wins - I share all the risks with the casino on an equal basis. It means that my investment part will constantly decrease until it'll disappear at all.
From the FAQ:
For example, say the bankroll's all-time high profit is 100 BTC, its current profit is 99 BTC and players combined lose a total of 4 BTC in a round. 3 BTC of that is net profit (exceeding the previous all-time high), so bustabit would receive 1.5 BTC and the remaining 2.5 BTC would go to the bankroll.
It sounds like they are only taking their 50% profit on new net profits. So when the profit goes up 4 from 99 to 103 and the previous highest was 100, only 3 of the 4 is subject to commission. It's like the site remembers the high water mark, and only charges commission when that high water mark moves up.
I think that makes a difference to the mathematical expectation, leaving it positive.