So, if you have the money to pay the fine without breaking a sweat, you just ignore the rule.
> Regulating it by neighborhood will allow people to keep drug abuse away from their kids
No, it won't. For much the same reason local fireworks bans don't, except drug use is a lot easier to conceal.
> Neighborhoods that do allow drug use will pick which drugs to allow and be able to tax the activity
No, they won't, for the same reason neighborhood bans don't work. It's impossible to police effectively.
Maybe areas that allow drug sales will be able to tax (some of) it (but any tax in the “legal” area directly reduces the impact of fines in the “illegal” area, and even people who go wholly legal will shift activity to the lowest tax areas available; you might also look into what a big business cigarette smuggling to avoid taxes is.)