Market price does not dictate the Actual price of an item. Even if we are talking about Market price, that is only a relative value and not a true cost of a firearm. Market value is what people will pay/sell an item due to the current conditions, and it always fluctuates unlike the actual price.
For an apples-to-apples comparison, take a Colt6920 (5.56) and a Smith&Wesson M&P15-22 (22lr). The Colt's normal price is about $1100, the M&P is about $450. Currently, Both have ammo that is almost impossible to find locally.
There is no way that the market price of a base M&P15-22 will surpass the Colt6920 based on ammo prices alone.
Would a piece of coal be worth more than an emerald jewel? Maybe in a million years, but thats not really an ideal situation either.