JD,
Why just focus on free sites? If you build and submit one free site a day, you should have plenty of time left to build some sites for search engines where you can be more aggressive with your sales approach.
It does take some time to get listed in the search engines (sometimes 9 months to a year) and you will be spending some extra time doing link trades to hopefully increase your position in the serps, but this is traffic that keeps on selling for years.
Don't just focus on one thing. If for example you use an "exit" link on your free sites you can exit to a nice mainstream and search engine friendly site of your own that has adsense ads on it. All those clicks that pay anwhere from a few cents to ??$$ adds up real fast.
