If you can find the specials when they have them Jenson USA and Supergo are probably the cheapest places with decent selections. A while ago Supergo had Maxxis mobsters, 2.5, 50d for $19. I've never paid retail for tires as I can't believe they cost almost as much as car tires
But those 2 places usually are known for being cheap and having great sales. Just check their sites and see what's on sale and such and if you're close to one I'd say just pop by and see what you can get.