How much is a link to my app worth? 
Travis: I'd recommend against it, although I don't have experience.1) if they're reputable, they will just review you (not charge a fee).  Also recall that the FCC requires bloggers to include financial disclosures now, so that will lessen the impact.
2) if it's not a reputable blog/site, then you're not going to get much PR benefit
3) a lot of these sites exist just for PR, and will eventually get revealed/shut down (there are few/no guarantees that the site will exist shortly, or that it will continue with any positive page rank.  Chances are they built their PR on greyhat techniques)
4) Doubtful that you'll get any meaningful signups through this, which means that you're just thinking about your PageRank.  Which leads to,
5) don't manipulate your PR; it will quickly become a full time job.  The proper way is to follow all the google guidelines and build longer lasting relationships with legit bloggers and review sites.