The local "Mietspiegel" might give an indication. It's an overview of regional rents created by municipals in cooperation with tenant and landlord associations.
However i doubt it can be queried online.
As an alternative you might consider checking what Immobilienscout 24 has to offer:
They are creating "Mietspiegel" statistically on the basis of their online platform.
http://immobilien.meinestadt.de/butzbach/marktspiegel
I have entered:
Where: 10km around Butzbach
Rooms: 2-3
Size: 60-120
and got the following results (€ number is cold rental cost)
1 offer with <4€ per m²
6 offers with 4-5€ per m²
28 offers with 5-6€ per m²
32 offers with 6-7€ per m²
13 offers with 7-8€ per m²
4 offers with 8-9€ per m²
1 offer with 9-10€ per m²
1 offer with 10-11€ per m²
Experience shows that offers on Immobilienscout 24 are a bit more costly than found in local newspapers, so subtract a grain of salt from that figures.