I was advised by a professor from U Maryland (now at WashU) that getting hired in Political Economy is becoming increasingly difficult, so it would follow that schools would likely admit fewer students interested in the field. He's a very well respected economist and basically laughed at me and told me to rethink my interests when I mentioned the possibility of Political Economy as a research focus.
I left an econ PhD program with an MA (as a partial result of the above conversation) and am now applying to Poli Sci PhD programs. Of the programs you listed, I can say that econ is now the most competitive and has the most strict prerequisites. By that, I mean few programs will admit students without having demonstrated success in multivariate calc, stat, linear algebra, and real analysis. The Poli Sci admissions process seems to really vary from school to school based on fields and focus. The trend in the field is quickly shifting towards quantitative methods, so that MA should prove handy. Admissions officials at several schools have told me that they would like to require a math background similar to what I listed for econ, but can't yet, because their competitors don't have any such requirement. I'll bet someone could write a game theory paper on Poli Sci admissions and get it published. Hmm... maybe that's what I will do while waiting to hear back from these programs. Yet, at the same time, there are other programs at top universities that require maybe one or two methods courses and seem to prefer to admit philosophers. For that reason, it's important to match your interests to specific faculty and to apply to a wide range of programs.
I don't know about history or business admissions, but I'm sure there are people in this or other forums who could write about those. Good luck!