If I purchase, say, the Western US map app through itunes for $29.99 and then want to upgrade to the entire US or maybe North America, is the price difference the same as it is shown on itunes for the retail price of the apps or??? Is this even possible, or is it only possible to add maps via some other method? I don't want to be roped into an inferior setup if I want to add more coverage later, but also don't want to buy more than I may ever use up front. Anyone ever upgraded this app?