I don't know if this will help a whole lot, but if you cannot afford static advertisement on high PR sites, you may wish to look into AdWords's content ads (display on publisher sites). AdWords from Google costs fairly little to start ($5 activation fee, and then you set your own CPC and budgets), and you can even get AdWords vouchers to make the costs even lower. Google some times crawls big sites with the ads parsed for whatever reason (my suspection is the big sites are using the special integration code that they generate via xml feed or what not), and so you can get link backs from them without even realizing :)
I noticed my site on youtube, ign, and a few other big sites, and my URL is listed too (though, not a link). These entries are also in the google cache, so they may also temporarly improve your PR a bit.
Andy Huang
http://www.wiigamingtalk.com/ - Nintendo Wii gaming discussion board