Did you miss the part where I said...totally legal and unregulated?
Make it as legal and unregulated to grow and sell as tomatoes are and re-crunch your numbers. I think you would find that an ounce of pot would fall to the $5 or less price range.
Legal and unregulated (including untaxed) would likely mean those low prices. However, marijuana is more like tobacco than grocery-store produce. The government puts excise taxes on tobacco. Around here taxes account for 4/5th of the cost of tobacco products. It doesn’t tax produce, and even subsidizes much of it. Governments will tax pot, and taxes will be the bulk of the cost.