Is it a full vetting or just a two stage? And whereabouts (roughly) are you, as prices do vary by location?
For example, I paid £125 including callout fee of £35 to have a yearling colt vetted last summer. I am in Northern Ireland, and the vetting included all five stages (exercise part was loose schooling as opposed to lungeing or riding of course) but no bloods were taken. It was really just to make sure he was sound, not blind and not a rig, but it also let me see that he was good with the vet!
So to me, £300 seems like a lot, but it may well be standard where you are!