I'd say you're getting ripped off for that price. We could have insured me (26) and Amy on a Subaru Legacy Twin Turbo for less than that, quicker than an Impreza, and about the highest insurance group available - it's the same a a Ferrari from what we could see talking to insurance companies. I have 7 years no claims, but after 5 years I think they stop counting.
Unless, of course, you live in Mosside.
The system of expensive road tax but including 3rd party is great, but it does mean that very young drivers can get access to very powerful cars and do a lot of damage. At the moment you could buy an Impreza for £1K on auto trader, with high mileage and absolutely knackered, but still extremely quick, and in the hands of an 18 year old extremely dangerous - they simply haven't been driving long enough to handle a car that can do that, and don't have the maturity not to drive it like a rally car on the B roads around. I'd rather have teenagers driving 1 litre tiny cars that they can do a lot of damage with, but the chances of them doing it are much smaller. In Denmark, having spent a week there a few years ago, you'd struggle to find another person to crash into on most of their roads!
Endoman, I reckon you could buy him something like this and have it insured for £500 a year at most. You'll probably make the difference back on the insurance in less than 3 years, and your son will have a "cool" car that looks fast, even if it actually really isn't:
For Sale - 1.6 1983 Ford Capri Cabaret - Capri Power Board maintenance cost almost zero, parts very cheap and easy to come by, and you can do 99% of the work that ever needs doing yourself. It's a nice car too, and stuff like this just isn't selling in the current market where people are ditching weekend cars all over the place.
Digs.