I think you just pay what you think is ok... I sold my Ibiza Cupra MK3 to a 19 yr old, the car was highly modified, but he didn't mind, and told me it was going to cost him over £2k to insure! Each to their own, fair play if they want to pay that.
Have to say, it doesn't get amazingly cheap to insure a GTI ... Im 29 with 7yrs NCB and can't get a quote below £500 FC! (Tried all the usual places!)
I think the insurance companies must think I live in a ghetto