I'll find out for you but will need to ask my brother when he's next out for a pint - shouldn't be a long delay!
As I understand it's a fixed price on the basis that you're only driving any one vehicle at a time (or two if there are two people on the policy), and so last time he added a bike there was no extra charge. They only asked if the value was less than £25k (it was!) as that was how it was banded.
I don't think the policy is cheap, except it's cheap by the time you've individually insured a number of bikes and cars.
Will let you know what he says.
Andy