It's mass produced for cough syrup, so you can get like 2 grams for under five bucks, and to fuck your head up as much with acid, you'd need like 3 or 4 hits.
It's totally legal to buy large quantities for "research purposes" too.
http://www.gdcada.org/statistics/dxm.htm
or
http://www.erowid.org/chemicals/dxm/dxm.shtml
The second link is fairly unbiased, where the first one is an anti-drug site.