It depends on how bad you want it. Typical PR 3 links with no traffic can go for about $5 to $10/mo for those pagerank hoes. Personally I would use the newness to your advantage and offer a set amount for a permanent link. If the site does not have any sold links then you could walk away by offering $50 for a permanent link. If they have sold some links you may need to up the price.
Bottom line is go with your gut feeling and make an offer for a permanent link.