There is often a price difference, but it's not really due to some inherent difference between NTSC and PAL.
The reason is that games are widely distributed in some areas than others, and Gas Hog just happened to be fairly widely distributed in countries with PAL TV sets, whereas it's almost impossible to find in most NTSC areas. For some reason, the cart is actually not too hard to find in some parts of Ontario, Canada. In the last 3 years I've picked up about 10 NTSC versions.. traded all but a couple of them to pick up 2600 carts which are impossible to find in this part of Canada.
Hope that answers your question,
Mike