Dude, where' my tax dollars?"---The State of Marijuana

The State of Marijuana | The Marshall Project:
"...Can't you just tack on a hefty tax? 
You'd think that would curb heavy use.
It might also give other states an incentive to legalize.
Colorado now has a year’s worth of data that shows significant tax revenues from legalization and very little negative impact in terms of crime.
The question is, "What's a hefty tax?"
Washington taxes cannabis 25 percent at the farmgate, 25 percent at the processor, 25 percent at retail, plus sales tax.
That means that about 40 cents of the consumer dollar goes to the state.
Sounds pretty hefty, no?
And it is, as long as market prices stay high.
But as market prices plunge — shops in Spokane now feature $200 ounces — the taxes plunge right with them. 
I think we'll see farmgate prices below $2/gram for medium-grade cannabis (compared to about $10/gram on the current illicit market) and retail at perhaps twice that.
Suddenly, the tax isn't nearly hefty enough to do what you want it to do: keep prices up and prevent big increases in juvenile use and heavy use.
The right way to do it is to tax based on THC content rather than price.
For 10 percent-THC cannabis, the right tax level would be $5-7 per gram..."