If you're looking at used bikes, look for one that's been raced by a halfway decent local racer. That sounds backwards, I know, but ANY bike will have been wrecked--it's the nature of dirt bikes--and racers are way better than the average spode about maintenance--ie, top ends, tires, greasing linkages, high-quality tires, chains, filters, oils & fluids, etc.
A mid 90s 250 should be in the $2-3k range depending on your area market and its condition. Depending on what type of riding you might want to do, 4-strokes are a safer buy; if they've been neglected, they can take a beating better. But they're heavy, and not NEARLY as responsive, and lack the finely tuned suspension of the mx bikes. But: they are way more forgiving, and especially if you are going to be on trails a lot, and maintenance is not as ciritcal. XR250s and 400s depending on your size and thirst for power are great, reliable bikes with lots of go-fast goodies available. And they're cheaper, last longer, and hold their value better.
That said, I'm getting ready to sell my XR200 (the Spode Chariot) and buy a KDX 200, which is a trail-oriented 2-stroke. I LIKE the 2-stroke hit but I don't want a pipey little 125 and a 250 is just too much at this point in my race career. And KDXs are cheap.