Firstly, PR, as far as I know, is no longer determinable if it even exists as a metric in Google's algorithm anymore. Second, if it's expired, can you just register it yourself? Usually expired domains are sold through auctions like Godaddy or Sedo, once they are truely expired anyone can register them. If you've checked the backlinks carefully and you can build pages to match those backlinks, it might be worth the price.