I normally base it on a combination of traffic and PR. If you are looking for link building you might be able to get a few bucks more. You would then sell links at an SEO site like Digital Point. Now if a advertiser is looking for traffic, 100 users a day, and that is not even uniques a day is very small. I would guess a link would get 10 click throughs a month at the most. So that value is not very high.