This is probably not the case in the US, but at least in Norway (and I reckon in the UK as well), insurance price is based on, but not limited to:
- Car model - if it's a popular model, often represented in crashes, it's going to push the price up
- Previous accidents
- Where it's usually stored
- How long you want to drive each year
- List price
So for an expensive and powerful car, with a young driver, it's going to skyrocket. Â£20k seems high though, but if he has a bad track record it's actually too excessive for those two cars.
I heard a story about a guy who had an older Ferrari. Insurance was pretty cheap - it was about 10 models or so in the entire country, and he paid the equivalent to a medium sized sedan - think Audi A4. Unfortunately, one of the other owners had an accident and had to use his insurance to cover it, and the next year the bill was quadrupled.