Hi dear fellows...
Directory submission service is one of the cheapest ways of getting many PR0-PR1 links. If you want to get 300 links from directories, you need to purchase at least 3,000 submissions (from our experience we know, that most directories won't add your link or will remove it with time or it wouldn't even be found by Google on their site; this is why you need around 3000 submissions to get 300 links).
It is difficult to specify how many "back links", but getting quality back links from high page rank site having different ip address and do follow attributes will help a site to get high page rank.