A State debt repayment tax on marijuana would be my idea? Marijuana you can tax with 50%. How much money could such a nationwide tax generate? What are the estimations of the DEA?
Not as much as you think since street pot sold in the informal economy would not be taxed and likely sold cheaper than legal pot that would reduce the income and further since you decriminalized it pot selling would not be a priority for law enforcement or prosecuted seriously. Colorado is having this issue now illegal pot selling is safer than before and as profitable as ever since non-taxed "illegally sold" recreational pot is cheaper so people buy it.
One way to tax users is to let people legally grow one plant free and if they grew 2-5 plants they could pay a $50/year tax. I'm not sure of the number of pot users but if there were 30,000,000 that $50/year tax would bring in $1.5 billion. A $100/year fee would bring in $3 billion in a year. It would be a win/win situation. People could grow their own organic plants that could use for personal use. If home grown, pot would be free of chemicals or pesticides. Marijuana bought on the street could have any number of dangerous things added to it. The marijuana could be treated with chemicals to induce highs or dangerous pesticides. Legalization would save billions a year.