If you have a blog, then you must consider these things in order to value your site.
1. How many subscribed users are there? If the number is big means more than 10 K then its good enough though in some cases 5 k is acceptable.
2. How many daily visits you get except your regular readers.
3. ranking in search engines
4. content, ( original, unique and of good quality), and
5. Ad clicks, this is something that is optional, but if you have a good click ratio per day then you can get a really good price for your blog.
6. Theme