I trained at Fairtex at Bangplee and at a camp in Phuket which was considerably cheaper and was better for tourism.
The training at Fairtex was a bit better and Fairtex had better equipment, but the camp at Phuket worked me harder. Overall, though, they were about the same except in Phuket, there's scuba diving, elephant trekking, island hopping, deep sea fishing, etc. The atmosphere at Phuket gyms are more old school.
I spent two months there and it costed about $4000 US including training, board, food and drinks, gym fees, and a week of touring the area (which amounted to most of my money anyway). If you're just training and not going to see Thailand which is a pretty stupid thing to do (you're in Thailand, might as well learn about a new culture), it should be around $2000-$2500 for two months of training including all necessities and luxuries. Of course, this is all excluding the cost of your flight. I fortunately got my tickets for a reduced price.
Just remember a few things.
Don't carry a lot of cash on you. In Phuket and in Bangplee, there are ATM machines that are pretty current with their exchange rates.
You barter for items at the store.
PLAN AHEAD!
If you're going to stay longer than a month, you're going to have to reapply for a visa which isn't that difficult but it's necessary.
Items are a LOT cheaper in Thailand than they are in the US. I got a good pair of gloves for about $5 US. It would've costed me $40 in the states.
Work on your conditioning. If you're well-conditioned, you'll be able to train more, train harder, and as a result, become a better fighter.
I suggest you go get an open water scuba certificate. It's a lot cheaper in Thailand to get one, but it takes around four days. If you've got 4 days to spare, then go for it. I didn't get to see or ride a whale shark, but the dive sites will take your breath away.