No cost for printing or book binding, true, but significant time and effort to create all the data within Roll20 -- what with maps, tokens, creatures, handouts, art, etc -- even more when it is a module whereby Dynamic Lightning is fully set up, the maps are integrated into Roll20 with correct scale (5'-per square), monster tokens are in the correct rooms (requiring one to read the whole module and room-by-place place every element). In reality, the DM is saved dozens of hours of preparation time. If the DM were to do all this, it would be a significant time investment. You could buy just the physical copy of the book, but then spend hours upon hours just putting in maps and getting everything ready. So the way I see it is you are paying not only for the book's contents but also for everything in it to already be set up within Roll20.