You will find prices on Gib to be the same as or even more expensive than UK! Also you won't know the price of an item until you actually enter the shop as the majority of shopkeepers don't exhibit the price in the window. Funny enough, every time I've been there its either rained or been very foggy!
If you do go, please enjoy, but be very aware of the Spanish customs on the border. They can be extremely thorough. I've seen coaches turned inside out when one passenger was found to have tried to take beer out of the Rock.
Personally (
and this is personal only) I don't like the place.