If you like him make them an offer you can afford, the worst they can do is say no. I also agree, for the age the price is a little high though especially these days. Has he been for sale long?
$750 is a lot for a colt, I have noticed that horse are more expensive in the UK then in the US. I payed £2000 ($3220) for my averagely schooled horse and the colts are fillies for sale that I see are around £200-300($320-480) a tb may be a little higher though.