Could you please explain how ROI is calculated then? I had understood ROI was "on what percentage I've incremented my bank in a period of time" while yield was "what percentaje I'm winning with each bet on average".
I'm asking because to me it makes a difference using smartbet since I'm paying a fee with each bet, so it wouldn't be the same a ROI of 15% being achieved in 10 bets than the same ROI being achieved in 1000 bets.
ROI or yield = (PL / staked) * 100. This is the STANDARD definition across all tipster platforms.
Costs that can be expressed as a % of your turnover can easily be factored in your calculations to determine the net ROI or yield by just subtracting them from the source's ROI. If, for example, you are following a source with 8% ROI and the source fees are 1% while smartbet.io's fees are 0.25% for the book you are using, then your net ROI is 8 - 1 -0.25 = 6.75%
All other costs that you incur by following a source (Cost of Investment as you put it) like opportunity costs, hardware/software costs, any 3rd party costs that you may incur, etc. cannot be determined by anyone else except you. You should use a suitable staking strategy that ensures that whatever time and money you invest is justified by your expected returns.
Finally, ROC is the return on capital over a certain time period and is related to your bank growth. If you start following a source with a capital of 1000 EUR and after a month this becomes 1100 EUR as a result of the bets that you placed, then your ROC for that period and source is 10%. If during that time your cost for following that source is more than 100 EUR, then you are losing money.