I was simply looking at products available on bodybuilding.com. You can probably find it cheaper from other sources. You also want to make sure they don't contain Stevia or artificial sweeteners like Acesulfame Potassium. They both can have negative health effects; especially when consumed regularly in large amounts.
http://www.truenutrition.com/p-1100-whey-protein-isolate-cfm... <-- 11.89/lb
Per gram of protein, this is far cheaper than organic chicken.
http://www.truenutrition.com/p-947-whey-protein-isolate-cold...
If you're willing to try Cold Filtration, you can purchase it for $9.40/lb.
This doesn't include the 5% off coupons that TN has year round. (AND028 is my personal one.)