We would say that depending on the duration of the tour, between 20 and 50 PLN sound fair. For tours up to 1,5 hours more towards 20-30 PLN and for two hour tours and more, something in range of 50 PLN will be much appreciated. Sometimes at the beginning of such tours, they will also tell you what comparable tours cost.
Here you can book different free walking tours in Wroclaw online - booking before is required due to limited spaces.