I'm gonna go out on a limb here and say... you are approaching advertising in the wrong way.
Assuming that you're looking for ROI from adveritisng, the formula is very simple:
- [Cost Per Mili] * [Total Budget] = Number of Impressions
- Number of Impressions * [Click-Thru Rate] = Number of Clicks
- Number of Clicks * [Conversion Rate] = Number of Sales
- Number of Sales * [Average Sale] = Total Return
The square-bracketed items are the variables that you can change, either by finding a different venue, improving your banners, tweaking your landing page, etc. That is how how you achieve a positive ROI from ads. You don't do this by trying to find a network where you can put hastily-made 125x125 banner ads.
Instead of looking for "dozens or hundreds of targeted blogs" look for a small handful of large sites (three or four tops) and invest in A/B (and even A/B/C/D/E/F/G) testing on different banner designs, landing page designs, etc. Once you find the sites with the right CTR and CPM, just keep throwing more money at them until the ROI starts to drop off.