Berlin is the only German city that allows street prostitution EVERYWHERE.
So you will find hookers parading even on the fancy boulevards like Koenigsallee (aka "Koe"). Try the Oleander-Strasse, it was crawling with them in July. Some of the women park their motorhomes or caravans right there. Prices vary and are negotiable from 25 or so to about 100 euros depending on the hoe's looks, time of day/night and your hornyness and gullability.
However, as everywhere, street action (aka "Strassen-Strich") is the least recommended and least safe form of prostitution, and not even always the cheapest either, since often you need to rent a room to do the act in (public sex is a big NO-NO even in tolerant Germany).
Try finding a nice girl operating in an apartment or a FKK/Sauna club like the Artemis (60 Eu incl food/drink + 60/session for sex). Much more civilized.