I haven't quite worked out the math here, but assuming the average price of a PR3 - PR5 link is $0.85/month. And if 10 of these links help push your site to a PR 4, that works out to around $102 per year. Let's take a network from BST for example, they sell $169 / $249 per month per individual, and there are probably 30+ external links of each site (there are many pages within a site that we can take advantage of too).
I guess the beauty of this is that you have full-control of incoming links to your private network. Suppose if the next update, a PR4 link dropped rank, or got removed. No problem, you can just purchase another PR4 link as a replacement and point it to your network again.
Everything is just a theory in this stage. I could be wrong but I always like to go creative in these things. Just sharing my thoughts.