It depends. If Google finds out that you're purchasing links, it will demote you. If not then sure, PR7 backlink would be nice. You still have to keep in mind that in SEO community PR doesn't count for much since it has no affect on your rankings. You can have a PR4 site outperforming a PR7. What does matter is number of relevant one-way backlinkgs from highly trusted sites, off-site SEO and on-site SEO. If you'd like you can PM me with your e-mail address and I can talk a bit more about on and off-site SEO.