If you view a blog as a business:
Income - advertising, affiliate links, paid placements, etc.
Expenses - hosting, and dev/design work, and the big one: content.
The income is related to traffic and audience. The more traffic the better, with some audience demographics being more valuable than others.
Expenses are dependent on how big and fancy the site is from an infrastructure standpoint. It can be pretty simple and run very economically. Except for that content part... how much do you pay your writers? Your own personal blog? Salary = $0, so low expenses. Paying for content... not $0.
A blog with lots of traffic has value. But that value will decline if the quantity/quality of content goes down. If I were to buy the MMM blog but couldn't entice MMM to stick around and keep writing, what happens? The traffic (and thus some affiliate/ad income) will be there for a while, but I need to figure out a way to keep the audience around. That might mean paying authors, driving up expenses, but hopefully building traffic and revenue. Or I might spend a little on paid promotion to bump traffic, plaster ads all over the site, and try to squeeze a few bucks out of it before I alienate the entire audience and flame out spectacularly.
To answer your question: Is it possible to determine blog value based on public information? Absolutely. Of course, accuracy depends on the quality of the public information. There are plenty of blogs that have sold where the price has been disclosed. You can use those as a baseline.
At the moment, I don't have the time to go dig into the details for DM or MMM, so won't work up an estimate. I'm sure some others will, so could weigh in once there are some concrete numbers to work with.
