The important things are the number of visits that you are getting and the proportion that are converting into sales/ad clicks. The most important thing to influence them is your position in the SERPS for the key search arguements. PR is just one factor that affects your position in the SERPS, it is not an end in itself.
Broadly speaking if many sites have the keyword "widgets" then those with the higher PR will do better for a simple search on "widgets". But if the thing that brings your sales is searches on "red widgets in Freedonia" then if there is little competition for Freedonian widgets the low PR is unlikely to be much of a problem.
You would then need to concentrate on content that identifies your site as being concerned with red Freedonian widgets rather than generically about widgets around the world and to concentrate on building links from more general widget sites and more general Freedonian sites. Remember that links aren't just a mechanism for building PR, customers really do follow them to visit your site.