There is no legal requirement that contracting parties give equal value in exchange. What you're referring to is commonly referred to as the "consideration" required for a valid contract, but it is easily satisfied. It does not require equal value, it requires merely that "something" be given by both sides. E.g., if the author of an article desires to have it published, a publisher's mere promise to publish could serve as consideration for the author's transfer of copyright. "We'll promise to publish this if you transfer copyright to us." That would be a perfectly valid agreement.
Also, you say "The authors pay Elsevier to coordinate peer review and publish using their brand names. This necessarily includes granting a license to Elsevier to copy and distribute the author's work." Well, transfer of copyright by author could be seen as simply part of the payment to Elsevier to coordinate peer review and publish the article. Consideration need not be, and very frequently is not, in the form of money. In fact, even in contracts where one party provides consideration in the form of money, the other party usually provides its consideration by promising to perform a service, or to transfer goods other than money.
There is a doctrine wherein otherwise valid contracts can be voided because of "unconscionability". Although the term is thrown about a lot, contracts -- even very one-sided contracts -- are almost never voided for unconscionability. The whole idea of contracts is that two private parties agree to terms to apply between themselves. Courts are thus very hesitant to intrude, unless there is a specific legal reason to do so, not just a general charge of 'unconscionability'.