Well by definition, if you have no profit your roi is NOT 50, roi is PROFIT/buyins, so no profit is 0 roi. To answer you real question, , 13 is no where close to enough games played to figure anything useful out, you need a lot more sngs played to get a real sense of how you are profiting. Hundreds if not Thousands of games.

ROI = 100*total profit/total investment

so if you played 13 dollar buyin sngs and made 13 profit, your roi would be 100*13/13, which is 100% return (you invested 13 bucks and made 13 bucks on it, 100% return)