The price drops but the number of units sold rises because it's no longer illegal. Just like lowering taxes generates more revenue.
Given that labor-intensive tobacco needs price supports and grower quotas to maintain a price point of 3.1 cents per gram, how is it even remotely concievable that fast-growing marijuana, without such price supports and quotas, will maintain a $10-$12 per gram price?
That just doesn't make sense.