A kilo of cocaine is lighter (1 kilo), harder to detect, more compact, and more valuable (150k street value of a kilo of cocaine) than an AR-15. The kilo of cocaine is sold to multiple end users, making the product fairly affordable for the end user. In contrast, the AR-15 would go to just one user, making it a high dollar purchase, which would of necessity be too expensive for most end users.
You mention the price of black market guns now as being fairly affordable. But the hypo we're supposed to be dealing with is their cost if guns we're all illegal. It would be substantially higher than it is now, pricing out most gangbangers.