Hm, since Canterlot is built almost entirely out of a sheer mountain side, whatever country land they have for growing is probably taken from the lower valleys and very sparse. So if Applejack and the Apple family lived in Canterlot, they probably would be one of the few ponies with fertile soil, which couldn't produce much fruit. Canterlot probably exports most of its food from elsewhere in Equestria, but certain noble ponies might prefer Canterlot-grown food, which would be much more expensive considering its scarcity and the required resources. So Applejack would probably have a thriving business, if very select on its customers.