i remember in 2004 or so when an agency i worked for had a wikipedia clone running with adsense and tons of SEO which made 20k per month and basically kept the company afloat. I was a young junior dev and while i was impressed by it, it never felt right to me (which it obviously wasn't in many ways). As far as i remember this only worked for about a year at best until Google penalised those sites more and more.