Interesting, I've been doing just that for the last couple of years. It's been a mix of partial successes and total failures, mostly failures. I didn't want to go the 'mfa' route for obvious reasons, so I try to make these sites in to something that actually is useful, and that have a sense of community about them. Here is a breakdown of what you can make this way in a month based on adsense alone after several years…
Personally from experience the hard work is in the research of finding a micro niche, once you have that nailed down then you quickly understand what topics are likely to be winners and which are to be duds. An understanding of SEO is critical though.
In terms of someone setting up, the barrier to entry may seem minimal but theres nothing like getting out there and giving it a go;
Google Product Search + Market Samurai + Hosting&Domain + X Site Pro + Adwords Account + SEO = Done.