Edit: Also, how you computed readership numbers for a blog?
As for computing the traffic numbers, first look to see if they have a media kit on their page or an RSS subscriber badge. Then look at the number of inbound links on Technorati and Google. Then check sites like Alexa and Compete. Also check the number of comments on the site, if there are any. Pay attention to the quality of the posts themselves. Are they writing original ideas, like Fred Wilson, or are they mostly regurgitating the results of PEW research studies? Have they won any awards? And lastly, take into account what platform they're on. A Typepad blog is going to have different traffic patterns than a Tumblr blog, simply because of the ecology of the platform and the userbase.
Using this you should be able to get a pretty decent estimate of the readership, especially once you have 10 or 20 blogs where you have exact data to use as a benchmark.
Our pitch email subject line was a brief description of the product then "- Free demo codes for your readers". The body had a similar format: what we do, here's your code, contact us with any questions, thanks.
Lots of sites have feedburner badges to give subscriber counts. We found one blog that was clearly faking that though. Their traffic to us was way lower than an 80k subscriber blog would send.
The number of comments on posts is a good indicator too. Anything greater than 0 means they've got a fairly big audience.
Nice hack and something I appreciate because it's such a simple thing to do.
"... Lots of sites have feedburner badges to give subscriber counts. ..."
Did you try to obtain the stats from each site?