You can buy websites for 10-12 times monthly net earnings. That means they pay for themselves in a year or less. That's a pretty decent ROI
. If you can add value by improving content, better ad placement, etc. it gets even better.
Sometimes I have found it is cheaper to buy a site so you can control its outbound links. That can make for cheap advertising.
Sometimes you can buy a site related to one you already have and test a new niche by linking to it from your existing site.
As a former real estate investor the part I like best is if you decide it is a mistake you shut the site down and it goes away. No contingent liabilities, no EPA lawsuits, and no exposure for being in the chain of title.