Methamphetamine is already legal in the US. It's Schedule II, same as most opiates[1]. (The illegal stuff like LSD and pot are Schedule I.) The brand name is Desoxyn and it's approved for obesity and attention disorders. Breaking Bad did another social injustice by not mentioning this. (Plus it'd have been an interesting plot: diverting such meds, hiring engineers that work for the manufacturer, stealing production designs, creating brand recognition, then selling counterfeit products (happens a lot to opiates), etc.) There's even a generic brand selling it! A quick search says meth tablets did $10M in sales in 2009; I suppose that's rather low and might be why people don't hear about it as much. Plus I'm guessing the DEA gets triggered by doctors prescribing it, even more than oxy?

Edit: Mayne pharma 2014 did 4% of its ~$113M in US products in methamphetamine. So that's 4.5M from a single generic maker. Could mean the total is well over 9-10M annually.

1: Including hydrocodone, except if you mix hydrocodone with a liver-toxic drug like APAP, then it magically drops into the lesser Schedule III. Because screw people.