I like the bargirl analogy...you're right Bob! It IS like asking about a bargirl...
Do a little research first. Go to the poor provinces and find out how much a virgin or low-use bargirl would cost. Many of the parts on a bargirl come standard. (NOTE I said many, not all)
You could ask a mamasan how much you could get for an old minger bargirl traded in on a younger, better looking 18 year old version. Ask her what would be a fair price. Probably somewhere in the middle of those two would be what to expect.
Then, checkout the bargirl herself and verify her condition. Make sure there are no leaks. Compare her with some of the other bargirls in town. Take her for a test ride and have a doctor check her out. (maybe do the latter first!)
And finally, is a bargirl really what you want? Bargirls are only in it for the money, even though they may act like they love you most of the time. C'mon, be honest here!
And remember, if the bargirl is native to Thailand, any foreinger can ride her, even if they are only here on a tourist visa for a short visit. Is that what you want to happen when you are away?
Take the time to ask yourself how long the bargirl would still interest you, and the cost per year. You might discover it would be better to find a different one after a while because you will get bored and the maintenance costs may get too expensive.
It is a different market, but the final price is still "up to you"!
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PS: Here is some more serious info stave off the "forum police":
I looked into buying one new (a Phantom) in Nan and they wanted about 84,000 baht, back in November 2005. They are a reliable and cheap bike but do lack in power, well, it's just a 200. I rode one hard for 3 weeks over all the mountains I could find and managed to only wear out the rear brakes. I can't believe the engine didn't blow! - Just don't buy a used one from a rental place! [

] I would consider the markdown percent on other used bikes in that year and km range. If you ask a dealer how much you could get for it as a trade-in they will low ball you, but that could be a starting point that's low for the private market. If you offer say, 50,000, (how much are they asking?)the worst they could do is say no.