Here are the stats about the feed subscribers to these blogs who use either Google Reader or iGoogle.
To interpret these data, you should know that:
The web visitors are not counted.
The PC-based feed subscribers are not counted.
The Web-based feed subscribers who use other feed readers than Google are not counted.
In the case of Marginal Revolution, which was one of the defaults proposed by Google Reader until recently, many feed subscribers do not actually read this blog.
A vertical blog like Midas Oracle, which deals only with prediction markets, is necessary far less popular than more horizontal blogs, which can publish about everything (including sex, as it has been the case in the past with those 3 economics blogs).