Like Kevin Garnett, anything is possible
I've worked at an agency, and interviewed at another, where remnant banner inventory or retargeting on sites and networks was purchased on a CPA basis. I haven't done any, and I don't really dabble in the media buy space, but it's possible in certain situations.
Some of the CPA buys I'm referring to were done directly with large, well-known websites, as well. Mostly it was networks, but there were some direct deals there too.that has nothing to do with direct buying
don't waste your time with CPA on a direct buy.. simply not worth it. Since large website's have large inventory with very few direct buyers, they are usually more than happy with negotiating a CPM down to something that is flexible/works with you (definitely beats the ad network's CPM for unsold inventory)..
Tragically, our impression at AdTech was that CPM prices have been actually increasing in the last year. Most likely this is because more and more big advertisers are switching from 'old media' newspapers and such, to online advertising. They have huge budgets, and low accountability for how the money is spent, which means CPM has become even more inflated.
My only experience with media buys is with direct websites, and even then that only makes sense if their very high traffic or highly targeted websites (it takes a huge amount of effort to get anyone to put up an offer no matter how good). Don't expect that to be a viable scalable option, but is possible to find profitable media buys that way.
My usual word of recommendation is to always negotiate CPA placements and if they won't do it then find someone else. CPM has surprising awful conversion rates compared to SEM or SEO. It's best left to large companies with stupidly large budgets to waste.
This.Attitudes toward CPA vary from site to site. Many are religiously against for fear it will undermine their premium sales. Some will bash CPA publicly and then sell it to you in secret or on condition of anonymity. Others like Yahoo, will accept on CPA after you prove performance metrics with a CPA buy - so they in essence take no risk. Anyway it is possible, but a tougher sell.