A common misconception. Forgivable, but still quite erroneous. Rice and corn suitable for brewing is just as expensive as malted barley. Check your homebrew shop prices! Rahr Six-Row Pale = $1.50/lb. Flaked Maize = $1.50/lb. Corn Grits = $1.49 (at my local grocery). Rice syrup solids = $4.99/lb.(!)
It's easy to look at the price on the package and infer that price=ingredients. It's wrong because it's easy. In the first place, BMC can purchase ingredients with such economy of scale that they can profit from a $12.99 24-pack that includes a free t-shirt. Micros really can't, not even when they're large. In the second, brands can command a higher price than simple economics deserve. Boston Beer is large enough to buy on a scale close to BMC, yet a 24-pack of Sam Adams Boston Ale still commands a "micro" price. Why? Because you'll pay it
Rice and corn lighten flavor and color. That's why they're used. When your market demands a mere alcohol vehicle, the less flavor, the better.
Moreover, use of grain adjuncts requires special brewing skills and specialist equipment. Remember that Light American Lagers use up to 50% unmalted adjunct. If you've ever mashed such a grist, you know the extra steps you must perform - and how difficult it is to successfully brew such a beer!
It's easy to dismiss BMC as crappy garbage unworthy of our attention as beer aficionados. It's easy to dismiss what you don't fully understand.
Cheers,
Bob