Generally on Flippa is that sites sell for more when they have at least one of the following (preferably more):

  over a year old
  high PageRank
  lots of inlinks
  steady profit
  lots of unique content  

If you can buy a site with decent traffic and PR for say $300, take 3-6 months to put more content on it (50 x $2/article) , build backlinks ($50), and do a little PPC to increase traffic and Alexa rank ($100), revenue and profit should increase to the point where you can sell the site for about $1000+.

I sold a website on Flippa for $90k. The site sold a single WordPress plugin I wrote and was selling for $97 per license.

Very cool, how long were you running the site before you sold it?