I think it depends on where you're at, what is available through your distributor and how much you buy. The cheapest in my area may not be the cheapest in yours. You'll just have to shop around and also keep an eye out for group grain buys in your area.
Briess is the cheapest I can get here. I usually pick up brewers malt or pale malt. Depends on what is available. I also recommend reading post #2 again.