Initially go up by around 20% due to excise and state taxes from what I understand. But then you have the indirect effects of the costs of compliance which will probably drive prices up just after that, but increases in volume, investment, and the willingness to trade profits for market share might drive it back down....I suppose it's anybody's guess in the mid to long term.
One thing that will change with respect to the price is that it will include STATE TAXES for the first time since prohibition. While patients in the medical system will be exempt from these taxes (provided they get a specific form of ID) the current taxation scheme will add $9.25 per ounce on cannabis flower and $2.75 per ounce on cannabis leaves (which is an ambiguity CalGrowers is working to fix...).
In addition there will be a retail tax of 15%. These taxes combined, along with the implementation of a new robust regulatory system will surely drive the cost to consumer up.
in case you want to read more, The Motley Fool has done analysis of where the taxes collected under Proposition 64 will go. The Motley Fool also estimates that costs for compliance may run as much as $20,000 to $100,000 per farm.