well, with that it depends on if u want new or used
new will set u back $5000 or more, used, depending on condition, anywhere from $1000 (older) to $3000 (newer)
there are many choices, stick with japanese bikes, honda, suzuki, yamaha, and kawasaki, or KTM. anything else and u will regret it (ie crap)
125s and 250s are very different tho, a 125 is a smaller trail bike, like the yamaha TTR125, but 250s are race bikes, like a honda CRF250 the 250 is taller and much MUCH faster, im guessing u want to look for a 125 since ur new to this, go to ur local dealer and they will explain everything, show u what size to get, then look for that
cheers