willq - 2:38 am on May 22, 2011 (gmt 0)
As a rule of thumb, look at it as an exponential increase (there's obviously slightly more to it, depending on anchor text, number of other outbound links on the site / page the links coming from etc.).
1000x PR 0 links = 100x PR 1 links = 10 x PR links = 1x PR 3 links
(again, this is a rough guide, there's no real way to give an exact relation between links because it's all relative)