Audiobooks: How much more would you pay?

All three books in the Unlived trilogy are now available in print, digital, and audio formats. Total sales for each are running about as expected, except for audio.

To date, Raymond has sold the most total and audio because:

• It has been available a year longer than Shelly and a year and a half longer than Reckoning.
  
• I've encouraged those interested in the trilogy to first read or listen to Raymond, then Shelly, and, if they're still interested, Reckoning, to follow the trilogy storyline. 
  
• Raymond has benefited from far more financial marketing support than Shelly and Reckoning to this point.

As of today, audio accounts for 13% of Raymond's total sales, 15% of Shelly's, and 17% of Reckoning's, not enough on its own to justify the cost of producing an audio version.

That doesn't mean I regret offering audio, but I do question whether it will ever become part of the "cake" or, at best, remain a thin coating of "icing" on the "cake".

I don't control audio pricing; ACX does. Without a subscription, audio versions cost two to three times as much as print or digital editions.

What percentage more than a digital copy, the least expensive of the three formats, would you be willing to pay for books you read or listen to?