I need to initially sell 15 copies of a short story to recoup the ISBN cost. I'll further need to sell 50 additional copies of short stories a year to fund the website in its current form, or 150 additional copies of short stories a year for the plan that would let me set up a nicer storefront.
You probably don't think about it - I hadn't given it more than cursory consideration - but there is a lot that goes into a book beyond the simple writing of the story. Front and back covers have to be designed. ISBNs have to be purchased. Copyrights have to be obtained. Fonts have to be chosen, and sizes: page sizes, font sizes, margin sizes. Pages have to be numbered, prefaces and acknowledgements have to be written...I could go on.