You have 2 ways in which to pay for your build. The builder just charges for the labour. Giving you a written contract as to the amount along with a finish date. Plus penalties to be incurred if finish date isn't met.
Second the builder gives you a price to include everything including materials. I went that way and I really think the builder lost out. In fact post build he actually said that he should have put an extra 500,000 Baht on the price as he did it too cheap.