I sell ads here for $50: per month (only one left, the others are actually paid for)
Internet Business ~ ViperChill
A lot of them are repeat customers as well
In terms of personal development you can advertise on my new site (which already has 22k pageviews in the 2nd month) for $20 for a 125 x 125px sidebar ad
That site is here -
PluginID - Plug into your Identity
Let me know if you are interested