^^^
there are more important things than money to be gained via prohibition.
but on the subject of money. yes, it does cost a lot of money to put people in jail. it also costs a lot of money to build a jail. a lot of people need to be employed to build that jail, guard that jail, and provide meals for that jail. a lot of people are going to need a place to eat, sleep, and live whilst involved in one way or another with that jail. and finally, in order to maintain the industry created you're going to need a lot of 'criminals' to fill that jail and justify the construction of another.