The solution is to open up the market and the prices and profits of the trade will drop faster than the price of gas.
Two problems:
First other than pot, drugs are dangerous substances that no company could market given product liability laws.
Second, legalization will not reduce crime and will only shift it. Criminals currently gainfully employed in the drug business are not going to start earning an honest wage because they can no longer produce, transport, smuggle or sell drugs. They will simply shift to other endeavors such as kidnapping, carjacking burglary etc. Crime is all about making a lot of money without having to earn it.