It's probably worth more in Latin America than the USA. Also, depending on where the bike was originally imported to, it could be impossible to register it in the USA. If it's off road only, than that is not so important.
While selling directly in most of Cent. America and Mex, is illegal unless lots of paperwork nightmares are waded through, that does not mean it's impossible. Locals always seem to have "creative" ways to get this done.
So ask around at dealers, see what they offer. (Ask double over what you actually expect to get)
Maybe try in Mexico City? Or to a Bike Rental business somewhere?
A good used XR250R in the US is worth about what you paid, or a bit less.
Is your version the Euro version? Street legal with all the lights, signals and such? Some US states you may be allow you to register it, but maybe not. Certain US states are more lenient but lately US Federal EPA regs trump state laws. It's all in flux, making it harder to bring in "foreign" bikes or cars of any kind. They use the VIN to determine if it was legally imported to the USA. A foreign VIN may trigger a red flag.
No problem riding it around of course, just selling it on and registering it legally.