It all depends. If that site has been around forever and it still makes $0-$100 a month, then the answer is probably no. If there's a steady gradual increase in earnings, then, sure you should consider buying it. As far is the price goes, I don't remember the exact formula, but I believe it goes something like this: take the site's average yearly earning and multiply it by 3. Now, keep in mind that buying a site full of copyrighted material is not a prudent thing to do. Make sure the site has mostly original content.
Like qwikad has said, it all depend on the site itself in question. You want to look at how long its been earning for. Has the earnings been steady for at least 8 months or more? If so and the price is right then yes. But if the earnings are sketchy and not always consistent then its a risk you take. It's always a risk buying established sites though. At some point, you have to bite the bullet.
I would like to add on how much the cost of the website. You might want to consider how long before you can ROI and how much work is needed for the website to earn. If it's passive then good, but you should also know how to maintain the website to get a long term results. I know a lot of people who bought an earning website but after 6 months it's earnings declined since they don't know how to manage it.
Agreed with the ROI calculation, but If you really considered buying it, better accordance to the specific that you are interested in, so If the earnings declined in the future, you can easily tweak it to development and make it more profitable than before.