In all honesty, it's typically easier and much cheaper to begin with SEO and organic search traffic. No, you won't experience the immediate rush of traffic, but you will slowly pick up analytical skills and design knowledge that applies to all forms of AM. Also, SEO and PPC are beginning to go hand in hand more so everyday (with new QS algorithms etc.)
It's just my 2 cents, but if you haven't got at least a couple naturally ranking sites I would start there first. Might even want to stoop down to the adsense level to generate some passive income. I say stoop down only because there's always been sort of an "anti-adsense" feel around here, but, having an extra $150/day or so eases up the pressure of losing or winning with your PPC campaigns.