I created a directory in april (4 months ago) and it's been getting about 2000 visitors per month and currently PR1. It has a cool unique design and the script I bought for about $100 from PHP Link Directory. I haven't really monetized the site so.. http://enwebdirectory.com P.S. I just saw someone posting a similar topic and I thought it was interesting
http://createwebsite-easy.com/how-to-know-the-price-of-your-site/ Here you go , this blog do share lot of tools that will help to calculate your site. It help to give you more price opinion before you sell it .
I have actually used those services but I feel they are unrealistic as they say my site is worth thousands of dollars although I haven't monetized it or anything yet.