what kind of site generates 500-100-$1500/month in ad revenues? Or is the money made by having 300 sites each generating $20 a month?
Without a ton of experience here, I can make some suggestions where you might look to find the answer: SitePoint.com, DigitalPoint.com, and WebmasterWorld.com.
You could try shopping around as if you were going to buy a website, and if you ask enough of the sellers, I'd guess that you'd be able to get a ballpark figure of what types of sites generate that revenue.
I'm not sure whether 300+ sites is the answer, but consider that you'll probably spend more on hosting and managing 300 sites than carefully building a smaller set.
I'm curious to find out if it's worth declaring oneself as an LLC and declaring the costs of website operation as tax deductible. Would anyone know off-hand?