Facebook's FBX inventory is selling at such low rates that a handful of loose change can buy you thousands of its ad impressions.

The exchange, which makes Facebook's inventory available for real time bidding and retargeting, launched over a year ago but is still not commanding anywhere close to the already-low prices fetched by display ads.

"The inventory is incredibly cheap, put it that way," VivaKi president Kurt Unkel recently told Ad Age. "There are instances where ad serving might cost more than the inventory you're finding on there. You're getting into under 10 cents if you really are trying."

Unkel's claims are backed up by data released today by Turn, one of a limited group of Facebook partners with access to FBX inventory. The average cost per thousand impressions (CPM) on FBX, according to Turn, is a mere $ 0.45. Not only that, a whopping 22.5% of ads bought on FBX are purchased at a price between a $ 0.10 and $ 0.20 CPM, according to Turn. Display ads, by comparison, bring in $1.28 per thousand on Turn's platform.

The data was released as part of Turn's Global Digital Audience Report. The company collected the data by analyzing the billions of FBX impressions it claims to see daily. Turn has access to information about these impressions through its partnership with Facebook. The information is necessary for the real time bidding aspect of the buys to work appropriately.

Facebook, when reached for comment, did not offer one. But the low CPMs might not necessarily mean FBX is not succeeding. Online ad buyers are focused more on performance than views, according to the IAB, with 65% percent of media buys purchased on a performance basis. That said, while buyers are now purchasing FBX inventory at low CPMs, many might be backing the numbers into performance plans and could be quite happy with their investments.

"Advertisers will pay for value and predicted performance, whether it's inventory that's purchased through direct buys or via RTB," a source at another FBX partner told Ad Age in an email. "Their primary goal is reaching their right audience, which we're able to do at scale on FBX."

Still, Facebook seems unwilling to tolerate the status quo. Yesterday, the company announced a competitor to FBX, a feature called "website and mobile app Custom Audiences," which will allow marketers to place code on their websites and apps to retarget their visitors on Facebook without relying on partners such as Turn, AdRoll and Triggit which currently have access to FBX. Notably, Facebook will offer those using this latest feature access to mobile inventory not available on FBX.

"We anticipate that marketers will use FBX and website and mobile app Custom Audiences in different ways," wrote Facebook in a blog post announcing the new functionality. Of course, not having to share revenues with third parties has to be on Facebook's mind, even though the blog post calls the two "complementary." With ads selling at $ 0.45 per thousand, there's simply not much pie to be divvied out.