What is the best time of year (weather wise & cheapest) to visit thailand ?

Best time weather wise - December, cool sunny days and cool nights. Cheapest - July - August, lowest of the low months.

the only way I can see the replies I have made to this post is to click on "reply to this post"!

late october to early march is the best time in most parts of thailand, but not in all...in the south sometimes are better than others...depending on which coast...i love nov and dec

I've been during June/July/August and also Dec/January. I prefer the weather in Dec/January, however that is part of the "peak season" and the rates are higher.
Rates are cheaper in the summer months, but it is quite steamy then, and you can get rain. I recall the red flags being up (warning flags on the beach) so we didn't get in the water on Phuket at that time. And Koh Samui was OK for swimming but we had heavy rain a couple of times.
Thailand is a bargain in ANY season, though, when compared to most of the other places people like to visit! Even in Dec/Jan it will cost you less than, say, Hawaii.. anyday!
Carol

PS Right now it is very quiet in Thailand... few tourists... so there are bargains to be had. My dh and son are both there now and enjoying it very much.