Most major retail stores (office supply, electronics and department stores) sell the HP ink cartridges for the same prices (suggested retail price). The retailer's profit margin is low even at full retail because their cost from HP is so high! When you do find a sale on HP cartridges, it's very rarely more than $2 off. The only way to find HP-brand ink for significantly less is to shop online and, even then, the savings is rarely more than 20% off retail.
I have an HP Photosmart C6280 that requires SIX different ink cartridges (Black, Yellow, Magenta, Light Magenta, Cyan & Light Cyan). Instead of paying close to $90 for a full round of HP replacement cartridges, I've tried a few alternatives with very good results!
I'd suggest that you check out Cartridge World, who sell their own brand of compatible ink for 30-35% less than HP. It has consistently been high quality and the page yield has been higher than I expected. Go to www.cartridgeworldusa.com to locate the closest store to you.
OfficeMax also has a line of compatible cartridges, but I'm not sure if they're available in store or only online.