Hi,
There is no flat rate in france
you can find girls from 20 euros in bois de vincennes (african girls)
average rate is 40/50 euros for street hooker (in Paris)
Girls from the internet
External Link/Members Only average rate is 80 euros for 30mn
Be aware that if you get caught paying for sex it can cost you a lot. (1500 euros fine)
If you don't well Paris better call a girl who will welcome you in her flat or in a hotel.