I don't think you understand criminals and why they buy cheap guns. They don't by them because that's all they can afford. They buy them because they may have to toss them and they see no reason in spending extra for something they may have to toss.
If you raise the price the criminals will just pay what they have to to get guns and chalk it up to the cost of doing business. If they have to spend a little extra they won't care. They wopuld certainly prefer to buy cheap guns but if they aren't available they will buy what is.
Your argument might hold water if the criminals were lacking in funds but last I checked many criminal etnerprises are very lucrative.