This must be unique to Northwestern. The housing prices at other universities is listed on the residential housing page. Greek housing was no different than anyone else and were a little more since the houses weren't as dense as a tower block.
The only way I can see the Greeks at Northwestern getting away with this is if they covered all the maintenance and all the utility charges. Considering that most of those houses are old and paid for NU wouldn't have to charge anything beyond that.
I believe that's the case, and that they actually have to pay for ADA upgrades on some buildings if they haven't already. It's still the same kind of housing that other students pay for.
