The amount a click is worth changes from site to site and topic to topic. The price depends on demand from advertisers, the supply of advertising inventory available, the performance of the ads (measured as the click through rate or percentage of clicks vs ads shown), and a slew of other factors.
Pricing also depends on the way the advertisers are paying. Some bid through auction based systems on a Cost-Per-Click (CPC) basis and others pay on a Cost-Per-Impression basis (CPM). When advertisers pay per click they don't have to worry about their ad being shown 10,000 times before getting a click, they just pay for the click so it's based on performance. When paying on a CPM basis they pay for the ad impression regardless if someone clicks or not (could be considered branding).
The type of site is a big factor because that also influences the type of traffic a site gets. For example, forums are high page view sites but very low click through rate. Viewers are engaged in discussions and don't click as often as readers of content sites where they might read a long article and be looking for the next place to surf, click on an ad, and continue their browsing.
Lower page view sites often have higher click through rates. These variables rarely scale. That means a huge site will probably not generate as much as tiny site when it comes to revenue per page. This is a generalization because the real big sites have huge sales teams where small sites depend on ad networks.
Forums still have a stigma among advertisers as unsafe places to advertise because of years of misinformation given to advertisers by content sites. For example, I felt that Vangaurd would be a wonderful fit to advertise here on our community. Unfortunately, I was told they did not want to advertise on user generated content sites. The stigma comes from the old usenet and BBS days when forums were primarily unmoderated. Ad sales people would point out the dangers of having your ad next to some white supremacy discussion or other improper content. The ad sales people were representing firms who pay a considerable amount to write articles and wanted to show the value of the content and use
FUD on advertisers that user generated content was a dangerous place to advertise.
The tide is slowly shifting thanks first to MySpace and now facebook as the buzz has become how to interact with people through social media. Unfortunately most advertisers still don't get it and think that shoving ads in front of people is the best way to advertise on the net. What we have discovered is the best way for small businesses to use the internet is to show ads to targeted niches and join them in the discussions but don't try and be a sales guy. Just participate and develop a reputation among the members and when they are looking for that service they will come to you since they know and trust you. Recently we had some shills on another forum site posting as members and asking about a service, the service was also advertising on our site. We found out, I banned the accounts and canceled their advertising campaign, we don't want unethical dollars.
Sorry, I got off topic on the OP. The answer is, it depends on a lot of things. If you are thinking about starting a blog, do so going in that you are going to do it because you love the topic and not to make money. Spend a few years and probably 750-1000 hours building the site, getting links back from other relevant sites and you might start to make a few bucks. If you keep it up, in 5-10 years it could bring in some decent income. Research the niche first because some pay horrible and others are sky high. If you are an expert on topics where large class action lawsuits are happening those lawyers pay huge leads to find new people to include in their suits. So any of the topics you see on TV where they say things like "Are you or someone in your family suffering or died from mesothelioma, call us..." make huge money. I heard that mesothelioma once paid over $100 per click!