Weed would not be dirt cheap, even if it was legal. Fdd is right, there is a lot of labor and supplies needed to grow a top quality crop.
The best way to figure it out is to look at what it sells for in Holland. I imagine the prices here would be similar if it were completly legal. I was there about 10 years ago and can only say what it went for at the retail level. I don't know if it has changed, maybe someone who has been there recently could.
Every shop sold a 25 guilder bag ($12.50 us, it was before the euro) The top quality stuff at the time was strains like white widow and jack herer, your bag would be like 1.2 grams. The lowest quality stuff was Thai and Jamaican imported, bricked up sativas and your bag of that would be like 4 grams.
So if the top quality stuff was selling for about $10 a gram retail, that would probably be about $5 to the grower maybe a little less, so that would put pounds at $2000-$2250. Thats half of what it is now, but still enough to make some money.