Who says marijuana smokers see potent and less potent marijuana as interchangable except for potency?
Plus if you are going to smoke it. It would seem reasonable that you would smoke as little as possible.
Which is exactly why "wimpy dope" will NOT be the same price as "kickass dope"---so you've answered your own question.
But it is also not necessarily the case that the price of marijuana would be directly proportional to THC content. For one thing, people who find even one puff of "kickass dope" too much will prefer less potent stuff; for another, people who bake with marijuana won't care much about the amount of plant matter they use.