The title is kind of misleading, no? The author charged $18k for a "7 weeks adventure where I enjoyed free lunches, drove 50 miles everyday, and dug through emails." Which seems like a pretty appropriate price to buy two months of life.
But look, they occupied his time for 7 weeks full-time. He could have taken another project (opportunity cost) if not for this project. I mean, the inefficiencies of their business processes are not his point of concern - they occupied his time and they should pay for it.
i think most developers would be surprised how small a part writing code actually plays in making money in software development. If you look at a piechart of the effort/time involved in making contact with a client, contracting, delivery, and close out writing the actual code is maybe 15% of that pie.
The static HTML page is ancillary.