Of course it would be more expensive to police it, man-hours are not free. The obvious upside is that it would stop the unwanted be behaviors and not tax the good (tourism).
The is no need for prosecution, as fines are handles through an administrative process nor is there a problem to collect the fines within the EU (from where most tourists in Barcelona are) due to cross-border collections.
The plan would immediately have a ROI if enforcement costs less fines were less than the current costs for cleanup/maintenance. Furthermore current tourism/hotel taxes can just be changed to bar, liquor store fees and licenses, if need be and thus have zero net impact on finances.
Lastly, and most importantly, ROI should not be the primary metric when trying to change bad behaviors. The current situation externalizes the costs of bad behavior, whereas my solution internalizes the cost. Even if there was a real cost to my solution, it has a net positive benefit to the public good, and to society that's worth paying for.
> That's forgetting the impact on the life of people who actually live there in the short term.
What impact? Not many are likely object to less public intoxication and puking in the parks.
I'm not condoning any of the following, but: - you don't bring liquids to an airport - you don't go to Thailand to insult the king - you don't bring chewing gum to Singapore - you don't fly via the US due to the TSA - you don't overstay your visa
The point is, if you want to stop unwanted behaviors, you make rules and you enforce them, and eventually (most) people, even those that haven't been to your country, will know what isn't condoned and modify their behavior accordingly or stay away.