I think the costs in the game reflect the "unit replacement cost" for the united states military for that particular item/weapon. It's not the same thing as a price you find because these prices reflect the fact that weapons for the military are purchased via contracts (measured in thousands ane or years) from the manufacture and are usually in massive bulk.
Prices in the commercial markets will undoubtable differ because they aren't frozen by contractual agreements.
For example if the US goverment purchases 100,000 M16 for a set price and all of a sudden Colt figures out a way of reducing the cost of manufacturing the M16 the commercial market may have a substatial price drop but the goverment still pays the old price since that's when they bought them. The opposite can also happen where the commercial market is much more expensive, because difficulties (or greed) skyrocket prices to compensate for lost profit on a miltary contract where they underbid.
also keep in mind that prices in the commercial market (esp the resellers) can be VERY differnt. Think about it. If Bill gates buys a new dodge viper, and for some reason wants to sell it, he might sell it for 5 dollars, cause he doesn't care about the money, he just wants it out of his drive way....
Or if a Babages store finds out they're going to get tons of PS2s they might sell off all there other stuff at bargin prices just so they can clear out the stock room for the PS2's.
anyhow what i'm getting at is you can't compare a price you find with a contracters..
boom