I don't think there are copyright concerns here: the publisher provides a PDF of the content to Perusall (presumably after signing a contract in which they say that they have all rights in their content and grant certain rights to Perusal). Persusall then handles selling access to students.
As for cost, Perusall boldly claims in the first paragraph that their solution does not increase the price to students. I take this to mean that when they sell access on behalf of a publisher, they'll be selling for the same retail price that the publisher used to offer when distributing on their own. Perhaps the publisher even gets to determine the price when working with Perusall.
Obviously, you can only claim that there's no increase in price to students if we assume that the students were always buying a new copy of the book at retail price. However, as the article acknowledges, students often buy a used copy or download the book from "some foreign website" instead in order to save money.
As for other systems for annotating and sharing, the one that comes to mind is Hypothesis. It can used to annotate content available at any URL, plus local copies of PDFs, and allows for sharing between users (even of local copies of PDFs). There are some others listed on the web annotation page in Wikipedia. I think I encountered Marky somewhere once but don't anything about it.
Kevin
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