This is only try if you start by producing a print book, of which the ebook is an adjunct, and do all the work involved yourself without regard to the cost of your time. A properly prepared book whatever the format is going to have associated production costs unless the above criteria apply. There are editing costs, and layout costs (or formatting, for an ebook) and cover design and marketing and the cost of a website and shopping cart. Even Paypal charges transaction fees if you want to accept credit cards.

That’s not to say I agree with the mainstream publishers who act as if they had to do all of that twice to produce print and ebook. However, given I’d like my authors to benefit from their work, to say ebooks should all be low-cost or free is a bit of a generalization. I prefer to say ebooks should be reasonably priced, unless it’s the authors’ choice to give them away.