How to estimate the value of having followers

Let's say that I created a business that somehow created content or news about itself that was interesting for 100.000 people to follow on a regular basis. Is there any very rough way of estimating the value of this business solely on the basis of this information?

I know that anyone thinking like an accountant would say that this question contains too few informations to be useful, but I am not asking for an accurate estimate, I am asking for a method on how to estimate. Once this method is in place I can adapt it to a more specific scenario.

I also know that the final estimate will very much depend on who these 100.000 people are, how homogeneous they are, and what kind of market opportunity they repressent.

Here are some variables you'll need to estimate:

1. Engagement Rate - This is the percentage of followers who will click on one of your links, retweet, like, etc... A typical engagement rate of a high quality audience is between 5 and 10 percent.
2. Impression Value - Assign a value for each impression you get from your audience. How much value do you place on someone seeing one of your posts shared?
3. Conversion Rate - This is the percentage of visitors to your site who will take whatever action you're asking them to do. Typically, you want them to become customers (buy something, hire you, etc...) but it could also include them signing up for your newsletter, downloading an ebook, etc... Decent conversion rates tend to be between 2 and 5 percent.
4. Conversion Value(s) - For each type of conversion, assign a value. How much is a new customer worth? How about a new email subscriber.
5. Volume - How many updates do you plan on sending to your audience? Daily? Weekly?

After you figure these out, you can multiply the volume times the engagement and conversion rates and their respective values.

There is no inherent value to having any followers. The value of followers is contextual to the nature of the business. In his very good answer above @JonDiPietro lays out the necessary variables and formula for a traditional sales-based site. What he points out is that the value of the conversion -- meaning how much each follower purchases is essential for determining the value of the followers.

There are of course other business model which would result in different definitions for the words. Perhaps you will be selling advertising the the followers are in fact the product presented to the potential customer/advertiser. Then the value will include information about the demographic overlap with their targeted market. Depending on the advertising model, then conversion rates -- or click through rates -- from the follower base may emerge as equally important.

But in the end, the financial value of the followers -- like the value of a business -- is only what someone will pay for them (or for access to them). Since outside of the advertising platforms that barter "key words" in an open market-type model there are no secondary valuation determinate, like a stock exchange, the value is always determined behind closed doors between someone who has followers and someone who wants access to them.

You can use Google ads as a baseline for calculating value. Still lots of assumptions you need to make, though, each of which could be off by an order of magnitude.

Let's assume 100K users, each who visit 15 times a month, and visit 4 pages on each visit. So you have 6 million page views per month. Assume your CTR is 0.1%, and each click is worth 10 cents. So they are worth $600/month.
(CTR of 0.05% and 5 cents/click is $150/month; CTR of 1% and 20 cents/click is $12,000/month. So you can see it is very sensitive to your assumptions.)

If that sounds nice be careful because 60 page visits/month is an active user. In reality you'll have 200,000 visitors worth 1 page visit a month, 80,000 worth 3 page visits a month, 10,000 worth 10 page visits a month, 1000 worth 100 page visits a month, etc. I.e. find the integral of an exponential decay curve to get a better estimate of page views per month.

Tweeting/blogging/vlogging/facebooking/whatever is really just another way of sharing your message with others, i.e. advertising. The cost structure and media are different, but the end result is that your audience gets the message. Analyze the value of your communication effort using the same tools you'd use to analyze an ad campaign, namely, the variables that JonDiPietro calls out. Try to do similar estimations for other media (tv, radio, print, web) to get an idea of how much more (or less) effective social media might be for your situation.