I've never spent too much time looking into the general issue of internet advertising cost/effectiveness/impact/etc... either in and of itself or relative to print and other forms of advertising. But, from what I gather a standard cost measure is CPM - cost per 1000 impressions.
A week of advertising on the liberal blog network can be had for $3700 right now. That buys you about 7 million page impressions at a cost of about 50 cents per thousand.
Pretty cheap by what I understand are the standards of these things...
