0.55% (0.87% for roulette, due to larger house edge) of the amount bet is taken for the commission, leaving over 67% of the profits to investors (assuming average luck).

For example, if 100 BTC is bet on Plinko, we will take 0.55 BTC. With 100 BTC bet, statistically investors should profit 1.7 BTC, and 0.55 BTC commission will be paid, resulting in an investor net profit of 1.15 BTC in the average case.