My girlfriend and I lived on Koh Samui for over a year:
Rent: £140/month (Total steal, 2 double bedrooms, 2 en suite bathrooms, kitchen, living room, parking beneath house)
Petrol (bought scooter): <£10/month
We ate out every meal as it was cheaper than to buy in our own food, so that isn't reflected here but we were on a combined wage of around 65,000 THB (c. £1400) /month and we'd usually eat in an upmarket place once a week, have a big night out every weekend and have around 10-12,000 THB left at the end of each month.
Why on Earth did we leave