My friends that are middle and lower class don't do drugs, although they do drink a considerable amount of alcohol.
I went to high school in the city. It was common for kids to smoke weed after class and came to school drunk. As for the kids who went to school in the upper middle class town I grew up in, their problems included ODing on heroin. The kids that smoked weed might not have the most balanced careers, but that is a whole lot better than being dead because you ODed on heroin.
Of course that is all anecdotal. However, I think it is a major stretch to believe drug abuse has something to do with not making enough money.
The drug problem needs to be fixed first with ending prohibition. After that happens the vast resources being spent antagonizing a war can be redirected to figuring out how to fix the core problem -- people being more concerned about getting high than what the rest of their life is going to look like.