In my experience the prices intimated here are far too costly.
I built a 180 sq m house for 300 k. Single story, three bedroom, 2 bathroom, large living room, separate dining room, detached kitchen that includes a room off, external wet room, laundry, car port, and converted an old barn into better strorage facility. All tiled, decent fittings throughout, excellent wiring to Euro standards, down lighting, walled and lit, air con, rendered, painted, gazebo and a garden that's well on it's way to maturing two years on.
Currently I'm adding a garage cum workshop for a newly purchased Itan with facilties to make furniture. Basically, I'm concentrating all the wood working stuff in an organised fashion so we can cut, plane, joint, sand and varnish relatively in situ.
The key with all of this was doing it yourself. The cost of materials are dirt cheap and any additional bought in labour is too.
Really, it's Thailand and it should be cheap. But somebody is taking you for a
quoting those prices.