There are three methods for which you can purchase banner advertisments:

<OL TYPE=1><LI> Pay for a set number of impressions
<LI> Pay for a set number of click-thrus
<LI> Pay a sponsorship fee, where your banner is shown for n ammount of time</OL>

#1 and #2 are the most common methods used.

Last time I checked Yahoo! charges $10 CPM (that means 10,000 impressions for $100, which is Yahoo!'s minnimum).

I would suggest purchasing targeted ads. Find websites related to your topic, and contact them about their rates.