Its gonna be hard to find a useful one for that price, but possible. Look for old Honda trail bikes like the XR100. they are super mild mannered and reliable for tooling around on.
If you save up a little bit more, you will be able to get a pretty high performance 2 stroke race bike (125 and 250 2 strokes rip and can take on massive amounts of abouse). Avoid the 4 stroke race bikes-- they are about as reliable as a rented Lamborghini, and loud as effing eff.