I always keep an eye out for deals on used tanks to get that buck a gallon advantage. But, like everything else, prices have been spiraling up. Glass tanks are running $3 to $6 a gal. Acrylic tanks are running as high as $15 a gal. One of the least expensive places to get a new tank is glasscages.com. Check out their site for an idea of pricing.
BTW, plate glass has also gone up. I recently built a 135gal and the bottom plate, alone, was $127 for a 48x36" piece from a wholesaler.