Looks like a 1978 Yamaha DT175 or something close to that.
If that is a recent picture, it is in great shape and looks all original and low miles, probably mostly street miles, too. For dirt use, most people would have removed the luggage rack, mirror, etc...and the original front fender would be long gone.
As to value, probably not much. Best bet is finding someone who had one or really wanted one back in '78, and does not have time to restore one. That perfect buyer is going to be very rare. To a beginning rider who wants a cheap, street legal bike to learn on, maybe you can get $500 - $600 if it runs as good as it looks.
All this is just my guess, so good luck!