Numbers like that are not common, but are indeed possible. I say that from personal experience.
The most important advice so far in this thread is being sure to compare after-tax dollars when making the decision to sell or hold. Consult with an accountant who has strong experience in the disposition of intellectual assets in your country.
Another factor to consider: what would happen to the site if you were hit by the proverbial bus? Who besides yourself depends on that income stream? Do you have a succession plan in place so that someone else could step in, operate the site and make sure the income continued to benefit your family? If not, that would be a point for the "take the money and run" column.