Eh, give it time to saturate, maybe. As you say nenj, different supply chains et al, but given enough time for the ease of one to push the other aside...
That would be an interesting study, yes. I'd expect those to go up, the street dealers have to compensate their lost income somewhere.
Be sorta' interesting to find out if that's even possible, tbh, though. In a scenario where it actually is undercutting the ground level price. Weed tends to be one of the major-ish substances income wise, far as I'm aware. I wouldn't be entirely surprised to find out trying to avoid losses to legal production (assuming it was driving down prices) by upping the price on other stuff would start pricing people out of the market, such as it is, to the extent that's possible when dealing with addictive substances.
... 'course, if it does have that kind of knock-on effect, and it hits the illicit market harder than just the loss of the marijuana income itself, well. Mission accomplished? Kinda' one of the reasons you legalize, to kneecap street distribution on more levels than jut the weed.
E: Though in other news, the FCC's apparently voting on whether or not it fucks the internet, sometime today or tomorrow? Be impressive if they don't, considering the current head's less a person and more a sock puppet worn over the industry's dick.