Most of the time I have received poor results when measuring on a CPA basis, compared to conversion through other channels (cost per registration in my situatio

Most of the time I have received poor results when measuring on a CPA basis, compared to conversion through other channels (cost per registration in my situation). I have had a few great successes of bringing CPA under $2 for a very targeted key words. For example a recent advertisement for a campaign we were running on the artist "Ray LaMontagne" worked superbly, probably because the keywords weren't extremely popular and we happened to be giving away VIP tickets to his show the same weekend. So it was very low friction. As with any advertising channel, your mileage will greatly vary depending on what you're marketing, what you expect in return (branding, sales, site registrations?).