Smugglers mark their prices up for goods you can't purchase elsewhere; they have a monopoly, and they have to cover their risks.
For goods you *can*, though, the black market still has to compete with the normal market. If you increase the costs on food, nobody will buy from you (because the official sources are cheaper). Even if the official sources are underproviding, you still have the problem, as mentioned, that nobody can afford anything. If you charge two days' wages for a single meal, you won't be selling much because people won't be able to buy much. Not to mention your customers all die, and dead customers aren't repeat customers.
The flipside of that is that between the increased risk, the cost of actually doing your illegal actions, and the need to have lower prices, black market goods available to the hab dwellers are probably going to be significantly worse in quality, because the cost has to be cut somewhere. Not that your customers complain much, because you're their only viable source of food, no matter how bad.
This may well be more thought than was put into the pricing system, though.