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In short, tally up your total winnings, divide by your total investment (including vigorish) to get a percentage (or fraction), and subtract 100% (or 1.00) to represent your investment. The difference, which can be negative, is your ROI.
-100%: You never finish in the money at all. Every dollar you spend on buyins is lost.
0%: Dead even - You make back exactly what you've invested.
10%: You win $110 for each $100 you've invested in buyins.
Etc. I'm not up to date but I think about 30% is considered the theoretical upper bound against REALLY bad opponents.