When shopping for the cheapest protein powder for bodybuilding, please bear in mind that generally you get what you pay for in the protein supplement world. A gram of protein can vary in quality.
The cheapest protein powders generally contain mostly whey concentrate, which is the least pure and least expensive of the protein powders on the market. Whey concentrate can be anywhere from 30% to 70% pure, meaning that you would be getting less amino acids per serving than a more expensive protein such as whey isolate, which is more like 90% pure.
There is also the brand name to consider. A brand such as MuscleTech is a fantastic product, but they spend tons of money on advertising, including professional athlete endorsements such as Jay Cutler, and this cost is passed on to you in the form of more expensive products.
A company such as Ultimate Nutrition spends less on advertising, and as a result, their products are considerably less costly.
is made by a leading manufacturer, and is the "store brand" at Bodybuilding.com. This is definitely a higher quality protein, but priced among the cheapest protein powders due to the limited advertising and labeling costs. It runs about $45 for 5 lbs, which is about 61 cents per serving.
Optimum Nutrtion's Gold Whey is hands down the best seller of all time in the whey protein market. This is due to the super high quality and moderate price. This product comes in at $52 per 5 lb tubs, roughly 70 cents per serving. Buying the 10 lb bag does not save you much, if anything.
There are hundreds of others in between, so follow this link
to see them all.