Just got back from the exact places you went to. Koh phangan, you will need stuff all, buckets of booze range from 200thb to 400thb. Beers 50-70thb. Food is cheep as won't pay more than 300thb for a meal. I budgeted to spend $150 a day and went nowhere near it, even with the mellow mountain shakes.
Full moon party will be sick, best party I've ever been to. Little dance clubs/sections all up the beach, booze stands everywhere, fire rope jumping and basically the entire beach just lookin for some fun. Didn't see one bit of trouble while we were there for 4 days.
Patong is a little bit more expensive obviously but there's is plenty more to get up to along Bangla. $100 a day and you dont plan on shopping should see you living very comfortable. Went over with 1500 for 12 days and came home with money.
Enjoy it dude, I absolutely fell in love with the place, specially koh phangan. No pushy salesmen to shit you.