Compared to enormous costs of the War On Drugs, the cost change would be clearly negative. It is much cheaper to ensure pot is not sold to minors, not distributed in schools, etc. - just as is done with alcohol/tobacco now - that to have dedicated police force, court system and prison system prosecuting marijuana users.

Societal costs of DUI would not change much - people that are careless enough to DUI would DUI anyway, and legal pot does not add much to it - whoever needs pot, knows where to get it even illegally, and obviously if he doesn't care about the law while DUI, he wouldn't also care about the law before, while consuming. So while a minor rise in DUI is possible since people that smoked in home because they were afraid to be arrested may now smoke outside and then drive, I think it won't be very significant. Most people know how to be responsible.

Rehab costs would rise since more people which have dependency problem would turn to rehab now that they aren't branded criminals anymore, but I think it is a good thing, and the cost will be completely covered by the reduction of costs in policing, courts and prisons. Especially that marijuana is a relatively lightly-addictive drug, so rehab would not be as costly as for heavier substances.