HI globetrotter,
welcome to the forum.
Thats a good question. It is not an easy one or a cut and dry question to answer but I will give it a shot.
I have heard fees or salaries, (which is a better word to use) range from 10,000 bht to 20,000 bht per month and sometimes higher.
An attractive Thai girl, or should I say a women with a pretty or cute face, nice hair and skin, firm sexy body and a sweet personality would probably run at least 15,000 per month but it could be more like 20-30.
Depends on you of how much work she will have to put up with. Don't forget, that money is her monthly salary. You will have to pay for everything else like when you go out to eat, supplies for the house, medical things that come up, etc.
The longer you stay with them the closer they will get with you and become your loving mate. Don't THINK that just because you pay them every month they will fall deeply in love with you in a week or so. Thai girls take along time to fall truly in love with a guy. BUT always remember, THEIR FAMILY WILL ALWAYS COME FIRST NO MATTER WHAT.