Sure you can!
California's medical marijuana program collects around $100M per year in taxes on sales of around $1B. Participants may legally grow their own under CA law, yet they're willing to pay $300/oz and more.
Right, just because you could manufacture product yourself, doesn’t mean that most people will choose too. Anyone can grow tomatoes in their yard, but most still choose to buy them from the store for convenience, quality, availability, or other reasons.
Also, $300/oz is still less than the street value for the quality of stuff you get from a California dispensary. So, even with taxes and the business marking it up to make a profit, the consumer is saving maybe 25% via legalization.