A straight % which varies, some say 5%, some say 20%+ and everything in between.
My issue with this is that while it is ok to use it as a guide, ultimately one needs to take the name itself into account and other variables so in summing up this aspect I would say never buy based solely on a percential without taking other factors into consideration.
To be completely honest and a bit off topic but I have found among GOOD generic keywords and in alot of acronyms the .org is as good if not more desired and now becoming more expensive. I saw this trend start about 3 years ago and would consider the .org over the .net based on the natural traffic you are likely to see with one vs. the other.