I bought a 480×60 banner spot for my niche(aviation) social networking site at very popular airline image site for $75 per day.  I just did for one day to test and I reserver one day for 120×600 banner which cost me $150 per day(part II will be on this).

Here is my stat:

Ran on Friday, Got 599 clicks according to their stat.  My Site gain 20 new members and $5 from Adsense.  They had Adbrite and I tried adbrite spot which cost me $175 per week and it was kinda waste.

For Niche, banner campaign can be very good if you can find similar site to advertise instead of contextual ad.