You won't find a modern four stroke 250-450 at that price they are about 130-180k. An older two stroke will be cheaper. There is a 1999 CR 250 at Chiang mai moto-x for 75k. The class most race over here is the 125 (or 250 4 stroke) so there are loads of those, not as many larger displacement.
I have a Honda XR250 Baja for sale shortly.
It belongs to JUDDADREDD, the guy broke his leg in a couple of places, (you can read his reports on his trip/accident on this site), and he has now decided to sell his bike.
I have got to give it a wash/clean then will take some fotos and post the bike for sale on the GT Site.
The Baja with twin car size headlights
It has Cambdian plates on it, so obviously it has to leave the country.
We are just in the process of getting another 3 months extension for the bike, from Customs here on Phuket.
Mileage 15,000 kms, Juddadredd bought the bike with 4,000 kms on the clock