Even with taxes it would probably still be cheaper. It grows everywhere and has a short growing cycle. If it cost Company XYZ 50.00 per pound to produce, they could sell it for 100 bucks a pound, the government could tack on a 300.00 per pound tax, and it would still be below today's current market value.
And let's not forget, if it were decriminalized, lots of people would grow their own for personal consumption.
So yeah, in all likelihood most black market activities relative to pot would be gone.