An argument that doesn't seem to have the weight it should among the legalization crowd is the amount that would be saved if the Drug War is ended. This argument should have a particular weight with your conservative friends. Maybe they won't be sold on the fact that the Drug War is racially biased or the amount of people imprisoned by the Drug War, but their eyes will widen when they see the amount of taxes saved.
Ankur Chawla, a writer for State Sight, released astounding numbers for what he predicted would be saved. His numbers accounted for law enforcement expenditures, judicial expenditures, and prison expenditures, which pretty much sums up all the elements of the Drug War quite nicely. His final numbers for saving in Oregon was $90,613,650 a years, Washington D.C. could save up to $44,655,310 a year, and Alaska could save up to $19,499,920 a year. Imagine all that money still in the pockets of the tax-payer, or at least going to something more worthwhile.
I may not be the biggest fan of state-funded programs, but I would certainly prefer these funds going to some kind of scientific research or public aid rather than public imprisonment. I cannot even fathom $154,768,880 being wasted on something as ridiculous as marijuana prohibition.
Instead of fostering knowledge for the next generation, we are keeping them shrouded in ignorance with an enormous cost. Instead of fighting global hunger, we are making sure your neighbor isn't smoking pot. Instead of allowing citizens to keep the money they have earned, the government will use it for a bad doctrine. And instead of spending this money on the exploration and discovery of our universe, we will be putting you behind bars.