What do you think is an acceptable return on equity (I'm not sure what "equity" means in this sense)?

ROE is a financial performance indicator. They're generally in the single digits, low double digits is considered good. In a competitive market, they should converge to round about the same number on average, even across industries (module riskyness, correlation with market as a whole, and such factors).

The ROE of academic publishers is abnormally high, indicating some sort of market failure.

In this case, scientists do almost all of the work (write, typeset (mostly), referee) for free (or nominal amounts), so expenses are low.

The publishers charge libraries inordinate amounts for subscriptions (dollars per page).

And scientist need to publish, and they need to publish in high prestige/impact journals, and the prestige/impact is tied to the title, and the publishers own the titles, it is hard for competitors to enter.

any return obtained by legitimate means. what makes their gains illegitimate is the market manipulation combined with abuse of IP law. Their "license" (who needs a "license" to print pages of information to be distributed to other humans, anyway?) lets them "own" the IP that was created by funding public institutions. Thus, their gains are illegitimate. That 30% ultimately comes from taxpayers (which is then ultimately pocketed by shareholders).

You got downvoted, but thanks for asking this, because I didn't know either and now I know.