We did a project on this in work - whilst we could see the obvious logic in installing a hard top (more secure, more weatherproof, etc), it wasn't the increase in value that caused the increase in premium.
There is a huge market out there for second hand hard tops, and therefore there is more risk of that hardtop being stolen - the average hardtop is �1000, or as Norman pointed out, prestige vehicles are twice that or more.
We see an increase in the theft of these items during the autumn/winter months - and it's not just the cost of replacing the hardtop we have to cover - thieves don't neatly open the doors to the car and pop the top off. They gouge the body work, bend the doors, and rip the interiors to pieces - the average claim for the theft of a hard-top fitted to a vehicle can run to about �5000.