Not much at all. In fact, since the tiles are only generated once, they could stick a CDN in front and never have to worry about anything but data transfer costs afterwards.
That CDN isn't free all the content and traffic overhead has costs, CDN charges you for each request, and every byte sent. On a small (under a million users a day) application it may not be that much... but even ten of millions of users a day, it costs a LOT
Hell, why do you think google invested so much time into image compression tech to save a few bytes here and there.
I doubt itβs much, the whole OSM infrastructure used to be handled as a favour by one of the London universities. I doubt DDG Maps is operating at significantly higher scale, especially once you take into account they just have to serve static data.
