Also it really depends how deep and wide you go..
I mean lets say the "Sport" nIche..
A main site relating to all sports.. lots of subniches inside that..
football
baseball
hockey
and so on and so on...
Going deep and wide with related sites, blogs, eec, etc, using all these resources with adsense, and FUNNELING the traffic to the main site also with adsense on it..
This has to generate something, as long as you plan it right with plenty of fresh content all round.. and GET the traffic.. you should ONLY see adsense as an extra bonus tho.. really you need more eggs in the basket.
Even if each sub site earned only 10 bucks a day..and you have 50 of them.. you know.. mounts up..
Rob