Choosing a high class resort in Thailand is going to be just as, if not more expensive than anywhere else in the world.
Rooms exceeding US $500 to $2500 dollars are not uncommon.
If you think just because it is Thailand it is going to be cheap? It isn’t.
You are going to PAY for quality resorts in Thailand, the same as quality resorts in your own country.
Yes you can find a backpacker hostel, or cheap room with no AC for $10 to $15, that is not the same as a quality resort meeting your expectations.
A proper quality holiday in Thailand will cost you the same, if not more that many western countries in the world.
You can save depending on what time of year you travel to Thailand.
The high season Nov to Feb is very crowded and expensive.
However, if your travel in July the prices many be up to 40%+ lower.
Best to shop around for the best prices.
With that being said, US $400+ and up during the low season is not unexpected for a quality resort. A week there will sent you back 100,000+ thb, which is not cheap.