Hi Dan,
Firstly welcome to the forum. We were there in June we being me and the other half and we were going through £60 to £70 per day on average but that's not taking into account for trips etc etc, but that was eating out twice a day and drinks, day to day running costs. The only word of warning is the airport is very expensive, and also the rest stop's on your
transfer if you get one as not all do.
Eating out is a joy and you should never get a bad meal anywere as there is so much compation for you custom. We have been going for years now and the worst meal we had was just average if you know what i mean.
Getting to
Marmaris is a piece of cake, you can walk it in about an hour, depending on how many bars you venture into, you can get the dolmus and if its your first time to Turkey i would give it a go or the water taxi's are another alternative, not sure of the cost of the later two though but not megger expensive.
Hope this help's let me know if there's anything else i can help with because if i can't someone will be along soon to help,
Peter & Elaine