As someone who frequently browses used electronics - you're way off on the locked Macbook prices - you probably won't even get a locked iPad (latest 3 generations) for €100 - let alone a MacBook.
A locked 2020 M1 MacBook Air is going for $319[1] on eBay. Compared to $600[2] for an unlocked version of the same model on the same site. So my instinct of about half-price for locked device checks out