WAG time:
Looks like about 20% of the heavy readers around here see enough value in KU to sign up.
A bit higher than I would've expected for an indie-heavy catalog.
Now, Amazon seems to have about 50M Kindle users (out of 80 million ebook buyers) in the US and at least 20M active eink readers. You don't generally buy a Kindle if you're not a heavy reader but then there's heavy readers on the app side so a good bogey would be 30M potential KU subscribers or about 6 million actual subscribers. That's $60M a month in fees and $720M a year. Over time, some will drop out, new ones come in. Figure once the novelty wears off it'll drop 30% and say it is a sustainable $500M a year business. About double what Nook is reporting this year which is almost certainly more than Kobo, more than Google, and probably (still) more than Apple.
Even at half that WAG, KU looks to be the second biggest ebook distribution channel after Kindle sales. (Gulp!)
Depending on how much it cannibalizes actual sales (20%? More? Less? 80% of Kindle books are *not* in KU, after all) Amazon could grow their ebook empire by anywhere from 10-25%. ($200M and up...) About the size of Nook...
Or maybe not.
It's just a WAG...
(shrug)
It does look like there's real gold in them there hills.
Will we see an iTunes unlimited? A Kobo or Nook service?
Maybe Apple will buy scribd or oyster.