It’s probably not a net gain though (I guess?). Money not spent on weed would probably be spent on something else and taxed, unless we’re assuming nearly all money spent on legal weed would have been spent on illegal weed (maybe true??)
Relative to what? If weed was still illegal and untaxed, there are additional policing costs not taken into account here. If you mean relative to a world where nobody wanted to do drugs, then sure, but that's fantasy.