To start off you could try the domain appraisal forum right here in DP and see what experienced posters think. http://forums.digitalpoint.com/forumdisplay.php?f=61
compare to other blogs for sale on flippa.com. I really don't trust appraisal sites as they usually overvalue. You could also do some complicated financial modeling to figure out how much you think you can make from the site, but probably too early for something like that. I would compare to the prices of other blogs that were sold.
The values given by appraisal sites are widely optimistic. They are worse than real estate agents! Going by the sales on Flippa, a blog sells for between five and ten times its monthly income. But that is no guarantee that it will sell first go, you may have to try several times before someone buys it.
The following factors weigh in the value of your site: 1. Revenues (normally 10-12 times the monthly revenues if it is established) 2 Traffic 3. PR/Alexa rankings 4. Visual appeal 5. Ease of admin use/operations Hope this helps.