The value of a blog can be determined in a number of ways. The most important factor is the audience that a blog reaches. If a blog is aimed at a niche audience, then the value of the blog may be higher than if it is aimed at a general audience. Another measure of the value of a blog is how often it is updated.
A regularly updated blog has more value than an infrequently updated blog. Finally, the content of a blog can also be an indicator of its value. If the content is well written and informative, then the blog will have more value.