I have used historic data over a few leagues and previous seasons and have managed to consistently find a way of making a profit. However I have used Bet365 prices which I have used from footballdata. My aim is to to start laying every game on betfair. My concern is the differance in prices between bookmaker and exchange. For example if I can show say a 100 point profit over time by laying at b365 prices would i still be making a profit on betfair or would the better prices they offer seriously reduce the profit to make it not worth while?