Short version:
Easiest way is to earn money at home before you go I'd say, or if you're good you might make a living there fighting.
Long version:
I lived and trained full-time in thailand for 6 months.
This was at Chai Yai gym in Chiang Mai(northern thailand).
Monthly expenses was about :
150$ for training.
100$ for an appartment next door to the gym.
Food: 100$++
And it's possible to live cheaper or vastly more expensive.
Last two months I didnt pay for training as I was finally doing my pro-debut. Usually if you're planning to fight regulary you wont pay to train, instead they'll take 50% of your fighting purse(negoitable of course). The fight purse starts pretty low, and can/will improve depending on you, managers and venues.(I got 1500 baht for my first fight, and had to spend 2500 at the hospital afterwards

)
I had a friend there who made a nice living off fighting(he had a sweet-ass appartment, etc), but he was
good, and usually went fights abroad in australia, hong kong, etc where you'll get a much higher purse.