I dont' know if there is a link anywhere to this, but 'rent' for the purposes of the rent-a-room scheme includes bills. If you are renting out two rooms, then the max you can charge is €317.50 per room per month, including bills. As for how much you can charge, you need to find out what the going rate in your area is. It helps to set down the ground rules about cooking/cleaning/overnight visitors etc. from the word go, it's hard to go back afterwards and say that something isn't working out.