“The ultimate goal for opponents of marijuana prohibition is federal legalization. But any serious reform of federal marijuana policy will most certainly include a hefty federal excise tax as well in order to 1) help fund regulatory mechanisms; and 2) garner support from lawmakers who would not otherwise be disposed to reform. Oregon Representative Earl Blumenauer, for instance, has introduced marijuana reform legislation that would enact a 50% excise tax on
production.

Proponents of legalization understand that healthy sales taxes are a great tool for furthering their cause. At a certain point, however, high taxes will encourage an illicit market. Where is the line? It’s difficult to know for sure, but if a 50% tax were enacted on the federal level, the marijuana industry in a state like Washington would face at least $1.92 in tax for every $1 of product sold. Whether this level of taxation is enough to encourage a black market is
difficult to say.”

http://business.time.com/2013/02/25/will-high-marijuana-taxes-encourage-black-markets/

http://finance.yahoo.com/news/marijuana-dealers-slammed-taxes-110900353.html