I'm seeking online ad networks to display embedded HTML ads that allow
iframes, especially with standard dimensions of 300x250 pixels.
Additionally if they can provide geo-targeting that would be useful.

In my research the ad networks I've found require minimum
$5,000-$10,000 usage per month. This is beyond our budget at the
moment. I would like to locate ad networks with the least monthly
payment requirements regardless of the cost of the advertising itself.
I understand these may be smaller networks which is fine.