I'm not sure I agree with that. We were after Mangala for ages, and John Stones was also a very considered buy. Otamendi was a bit more spur of the moment (relatively) I think, but I don't think we've been panic-buying defenders, we just haven't been great at identifying great ones.
But yeah, I agree. We'd have to pay at least £30m to get someone as good as Mangala in this market and we'd probably get less than £20m back for him, so it makes no sense to bring another player in as cover, unless that player is far more suited to our system, or more versatile.