I'm no expert but here is a list of things you should consider in my opinion...
1: Traffic Stats - The more the better as it shows the popularity of your website.
2: Revenue - If the site gains some sort of income, the more people may be interested
3:
PR - Some (not all) like websites in which have a google page ranking
4: Content/Niche - People usually want to know the niche/content, the more demanding the niche the better.
Also, whether your website will be sold along with any scripts. software or content included, if so (depending on the script, software or content) the more people may want it if the price is worth it.
For example, If your site gets up to 10,000 unique visitors, gains revenue from google adsense of $100 or more a month, has a
PR of 3+, the niche is of high demand and the site comes as a whole which includes all designing of the site, scripts, software and any content...The better the want of your site from another webmaster in which the pricer the site should be considering the above.
Thats my own opinion of course and how I view it when I would be interested in buying a website. You may have an expert appraise your website as well to get a better understanding how the cost of your site. You may also place it on an auction as well to see the amount of interested people whom may want to buy your site.