I hope you're joking. $15 for any fiction ebook is not a sound business model. I'd buy a good ebook for $5, but not $15.
I can only accept prices like that for certain kinds of non-fiction works where the market is smaller and the production/compilation effort is way higher.
Paying $15 is paying to have it NOW.
If you wait to read it in a couple years, it will be much cheaper. Given the huge amount of written material available since the invention of the printing press, there is no real reason to read any fiction NOW, when reading it later will be just as entertaining.
Waiting a couple years or three e-books start costing closer to the amount of the author's royalties (if he was smart). I'm fine with paying a few bucks to the author. Maybe a few cents to the distribution chain.
Generally 3 to 4 bucks is what I like to pay. But Free is a good price a well.