Petcetera is really expensive - are there any small petstores around you? They usually have much better prices.
When I bought Orijen, it was $57 for the largest bag. And I know my mom pays $62 for Now. Those are the most inexpensive we've been able to find. What about Acana? It might be cheaper, but I'm not sure as I've never personally fed it. It's made by the same company as Orijen, but isn't grain free.