In the Past
There used to be lots of ex-pats living in Thailand, often on a very informal basis. Thailand is a country that has grown accustomed to over-charging tourists for everything from bottled water to Thai food to taxi journeys. Ex-pats have got around this problem by learning a few words of Thai and making local friends.
It used to be possible to get a series of tourist visas for Thailand by nipping over a border every 3 months. Foreigners worked in bars, dive shops and generally survived on a small budget in order to live next to a beach, in Bangkok or up in the north. Successive Thai governments have done a lot to clamp down on this. They have ordered their embassies in Malaysia, Laos and Cambodia to not renew tourist visas. They also made a few half-hearted efforts to raid dive shops.
Nowadays if you want to live in Thailand you have to marry a local, be sponsored by a company or get a pensioner visa. As a result many of the beaches in Koh Samui and Koh Phangan have lost many of their long term foreign residents.
Cheapest cost to live in Koh Samui
To live a basic life in Koh Samui you need a minimum of 15,000 Thai Baht. a month The cheapest rooms and bungalows have to be hunted out. They are normally privately owned places and can be had for between 3,000 and 5,000 Thai Baht a month.Most of these places are in the quieter south of the island. Don't expect to find them by searching on the internet!
As you can calculate, that leaves between 10,000 and 12,000 THB for living costs. The great thing about Koh Samui is that supermarkets such as Tesco Lotus make it possible to self-cater cheaply. The supermarkets charge everyone the same price.
If you have the funds to buy a moped (about $1,000) you can cut out most transport costs. Taxi drivers in Koh Samui never go on the meter. If you are on a budget they are out of the question.
On 15,000 THB a month it is still possible to go out occasionally to local bars and restaurants. Invaluable places for those on a tight budget are the numerous 7-11s on the island. They have cheap food, drinks and beer. They will also give you hot water if you want to eat a pot noodle.
Other than going to the beach most things in Chaweng and Lamai will be outside this budget.
Finally, it is worth mentioning that Nathon has the best markets on the island. They are good places to shop for household goods and fresh produce.