Unknown training and a kicking problem. Your location plays a key role as well. He'd probably go for 1k around here.
He has some cow kicking defiance issues but he doesn't seem to bad confirmation wise. Apparently he has good training but his current owner has no actual information on it...
Any vices? Bucking, rearing, girthy, pushy, not started well? Knock off a good thousand. Conformation plays a big role as well. Long sloping pasterns? He won't make a good serious riding horse once he hits his late teens. Sickle hocked? In at the knee? Steep shoulder? Again, won't make a good riding horse once he hits his late teens and the aches and pains start to catch up to him.
Well started horses with decent conformation can easily go for a couple thousand (in the leisure horse industry). Hope I helped some :)