You might want to take a look at what Google Engineer Matt Cutts has been warning folks about buying / selling links for a few months now:
http://www.mattcutts.com/blog/text-links-and-pagerank/
That aside ....
PRX does not mean traffic.
PRX does not mean quality.
Finally, they are paid directories that I don't think will be penalized in the long run by Google, and if I were worried about
PR, I would simply buy a link on a high
PR directory that's only going to run about $40-50 per year.
Good luck with your endeavor, but be careful what you ask for ...