I used to live in Frankfurt and here's some tips.
There are plenty of cheap places to get action near the main trainstation. Go to Taunus strasse and Elbe strasse. You'll see plenty of places to walk up. Just be prepared, they are usually multi-level and have very very narrow staircases. This will get you to the thick of things.
http://goo.gl/maps/6IqDK Cost of a typical f/s session is about 25-30euro. Oh...and be careful of some of the buildings with mainly Thai or black chicks...those Thai chicks are in fact lady boys.
Here's another street with a couple places to check for a cheap lay. It's at the other end of the pedestrian shopping street, Zeil. It's on Breite Gasse.
http://goo.gl/maps/m2jfx You can check out bulding numbers 27 or 29, and in between those two is a sex shop.
Do not, do not go into hostess bars. They will rob you. Ignore and walk past any bar that tries to pull you in for drinks.
For higher end/hotter looking ladies, check ouf the FKK's as mentioned before.