Well you are dubble lucky! Never, ever had a TG, BG allow to use the "backdoor"!
About your budget-questions it depends on many things. I rent a house in Kata (if you are there a longer time, you don't need always the crowd of Patong), where i rent a house with 2 bedrooms, kitchen, livingroom with tv/dvd, for 15.000 B/month. Don't get too excited, this is a nice house, not so big and no fancy things like a swimming-pool.
For the rest it all depends. As you know it smaller restaurant, food is cheap, but all these beers a day, could make expences go higher! Also depends if you take a lady every day, if you keep one several days (food drinks, little presents).
Anyway, a lot of fun can be had for a resonable price!
Welcome to the board!