The only model I'd have any interest in would be the UK model. The only "registering" one must do is to register themselves as a business for tax purposes. There are no "zones" and no regulations about where it can be offered, with the exceptions of "no soliciting zones" in some cities keeping the streetwalkers away.
In the UK, it is legal to buy and sell sex, but using force to coerce is highly punished. The only drawback to the UK system is that ladies cannot share an incall to save costs, as it violates the rules about keeping a bawdy house.
Rates in the UK are very comparable to rates here in the Dallas area. Most ladies between the equivalent of 200-300.. with a few outliers to both the high and low end.
One thing to keep in mind about the rates in Germany-- there are a ton of Eastern European ladies that have flooded the market, because to them... 50-60 Euros is a small fortune in their countries.. its much like girls from Guatemala and Ecuador coming here. The reason I don't see rates falling like that in the US is due in part to immigration laws (even the legal brothels in Nevada cannot sponsor someone for citizenship) and in part due to the cost of living here. I think rates may stabilize some.. but we won't see dips down at or under 100 for the most part.
There have been several groups actively working towards decriminalization and some working towards legalization along the lines of what Nevada has... thus far, the puritans have kept such efforts largely at bay.