Very expensive
My advice would be to stick to Darling for a Perv/Cheap Thrills only, and then save the mongering for other places and im not saying this just because of the high prices, just because they charge alot more than everyone else doesnt mean the service is going to be great.
Instead go in have a look, have a few drinks and a perv and then head to other places to do your mongering, as it will not only save you money and the buyers remoarse 30 mins after you've done the deed and found the girl to be brutally business driven.
Sex at Darling is ok to ask for, as alot of them do offer sex, but bare in mind some dont offer sex as i've heard from some people, its just a matter of seeing a girl you like and then asking her.
As for prices im not entirely sure, as alot of the people i know of who go there ask for extra's, and they have paid up to 5000czk for an hour which is pretty much what you will pay if you go to K5.
So its all up to you, whatever your comfortable spending at the the time.
Cheers
EuroLover