The most common "definition" of ROI in a punting sense seems to be your profit divided by the actual amount of money you have put into your betting bank.

For example lets say you start the year by putting $500 into your betting account. During the year you make no further deposits or withdrawals. At the end of the year your total bets have been $30,000 and your account now has $3500 in it.

Profit = $3500-$500 = $3000
POT = $3000/$30000 * 100 = 10%
ROI = $3000/$500 * 100 = 600%

So your "return on investment" is the $3000 profit divided by the $500 you originally put into the account which gives 600% ROI.

I agree with Chris though that this is a pretty useless measure so I wouldn't worry about it too much. POT and maximum drawdown are far more useful bits of information.