Curious about this one. The price is set by the publisher, so it's not like you can just opt to pay for a DRM or a DRM-free copy of the book at checkout.

I assume you mean that the publishers who offer DRM-free books tend to be more expensive? That might track if they're indie publishers, since they can't afford to be loss-leaders on a product.

But FWIW, Tor Books offers all of their stuff DRM free across the major platforms (Amazon, Kobo, Google Play, B&N, Apple Books, etc.) and they go on sale just as often as the major publishers, in my experience.