In which case I'd probably think around £3k depending on how much mileage he's done. If he's had a real rough time of it I'd be wary, personally. Also, if he's never done anything but polo he might be a bit interesting in the school and need lots of work.
It also depends on how good he is on the ground, out hacking alone, shoeing, etc. and whether he'll take your head off if other horses go cantering by or if he's sane in an open area. Stuff to think about.