Sex work is a particular sector of the economy, bikone. No government seriously legislates for taxation purposes. It's just not worth the effort to try to make sex workers pay their taxes, although some basic measures could yield some taxation revenues (to preserve a minimum appearance of equity). The costs of gathering the evidence for each and every sex worker far outweigh the benefits. Most sex workers enter the business as a short, temporary, time frame. Paying taxes for them, is pure lost. Also, there are those, like Maria Divina, who think the State has no business benefiting from their intimacy. I can understand that easily.
Cost benefit analysis should be made on the other elements that you point out: public health, policing and judiciary costs. Decriminalization or legalization have clear advantages IMHO. Health official can reach more easily the sex workers in case of STD outbursts; policing costs could be lowered if sex workers stopped seeing the police as their enemy and the judiciary system could be uncluttered of many stupid cases they have to pursue with no benefit to society. BTW, I'd be curious to know how seriously the public health officials are consulted at the moment.
The "Black Economy" ie untaxed money is spent in the community and stimulates trade, therefore Governments tend to turn a blind eye to lots of the "so called" black market" as its totally not cost effective to police it. Sure they will put some legislation in place, but it will not be rigorously enforced, unless it becomes a nuisance, or overtly obvious. As long as SPs and clients remain discrete there will be few prosecutions, (Just sufficient to satisfy the 'holy willy's' and give the police a little press coverage.) SPs and clients spend considerable amounts of So called "Black Money" in the mainstream economy. SPs need a healthy and safe environment to work in which should be the general area where the legislation should be directed. They have turned away from prosecuting recreational drug users as its non productive in terms of policing and controlling the problems. The same will happen with the "adult entertainment" industry a few high profile cases to make things look as though its being managed then it will die down.