ROI, as stated, means Return on investment. It can be easily misunderstood, however.
In order to get a good handle on your ROI, you need to play a lot of games, and see what your average is over a longer period. You can win or lose a significant amount in the short term with out that necessarily being a balanced view of what your ROI is. If you invest $20 in am STT, and get 1st, and that pays you $90, then your ROI is 3 digits. But this will not be a good indicator what your ROI is in reality. Just as it would not be if you lost that STT.
ROI is a long term calculation. It can of course be done after every game, but this will not give you significant insight to what you actual ROI is.
I cannot give you the best number of tourneys you need to get a reliable ROI, but for STT I think you will need 1000+ games to begin to get an idea.