Personally, I think the single greatest effect will be the savings and improved service of law enforcement as they can concentrate their efforts of serious crimes, and not petty matters of weed possession.  There are billions of dollars spent annually enforcing archaic drug laws and incarcerating otherwise law abiding citizens.
And as far as the tariff concept is concerned, most economists would argue against tariffs as they distort competition and generally lead to economic rents.  There is potential to enhance overall government revenues, but it has to be balanced against the allocative efficiency gains from proper competition.  It would also depend on the elasticity of demand.
And from a libertarian perspective, prohibition amounts to an assault on our collective freedoms based on unsupported allegations of social harm from right wing evangelist organizations and lobbyists for self-interested industries.  The prohibition of marijuana was largely effectuated by the tobacco, alcohol, and cotton industry whom all saw it as an alternative to their products (hemp is much stronger than cotton).
Most people don't know that hemp oil is an amazing industrial product and produces strong light weight materials.  If we open the production, we can obtain economic gains in a nascent industry by commercializing the processing of cannabis derivatives for industrial uses including pharmaceuticals, textiles, and plastics among others.
Prohibition is largely and idealogical debate that with prohibitionists closing their eyes to the enormous world of benefits that can be obtained with a more liberalized and pragmatic approach.