I totally agree with Thomas 69 and slutmagnet. The redlight district is not a total rip off.
It has a lot in common with London's soho district in that you have a few "strip club" rip off places where they try to get tourists inside from the streets, promising a lot of action but delivering little but high tabs. Avoid any of the places these places but go for the "eros centres" which are big appratment buildings with lots and lots of appartments with hookers standing in the doorway; this means you can look, talk and negotiate beforehand. If you don't like the girl, go to the next one, then the next, all in the same building. There are around 10 eros centres in the red light area, they seem to open pretty early, before lunch. Some ugly girls (mostly german) there but some beauties too, especially the south american and thai girls. Prices are mostly around euro30 for 20 min service, but if you give them 50 that will buy you some more time and better service. I had a very cute girl from dominican republic, around 23 years, with huge natural tits, totally shaved pussy, not the best service I have had but very good for 30 mins in a pretty expensive country like Germany.
Comparison with FKKs:
I had planned to go to one of the FKKs, but as I found the red light district girls so attractive I never did. The FKKs are hard to reach from city centre and they cost euro 65 in entrance and euro50 for a 20-30 min fuck. So I decided to not spend time in traffic trying to find some well hidden FKK club and save myself both time and money by staying in downtown Frankfurt. I know a lot of guys like the FKKs but I'm happy with my decision and I would recommend the red light district to anyone spending some time in Frankfurt.