I think you want the legal price to be somewhat below what the illegal price would have been. That should be adequate to get rid of the illegal market.
The thing that hasn't been mentioned here is the necessity for enforcement. You essentially have what's now a tax evasion market. If you have a high tax and don't enforce that, then you're going to have entrepreneurs come in and sell untaxed cannabis as they sell untaxed tobacco. If you're going to have rules, you need to enforce them. It seems to me that ought to be part of the planning for this policy. The prices should be lower than illicit prices, but not so low as to encourage heavy substance use.