Most of the players would likely agree to that release clause being a reasonable representation of their perceived market value though, none of them would have gone in to negotiations thinking that they would struggle to generate interest unless on offer at a significant discount would they?
Obviously you're taking about relegation so that skews things a bit when you delve into the hypotheticals, but the club and player would have begun the journey anticipating that they would appreciate in value, and so any proposal that included a clause where the player would be sold for around the same fee as was originally paid is already looking at things conservatively.
Whether these potential clauses also reduce year on year over the duration of the contract is another factor that we won't know until if/when the worst actually happens.
Again though, my main point is that it absolutely won't be as simple as all of our players just leave of their own accord and the club has no say in it, one or two might force the issue (Romero), some will be gladly sold by the club and will also most likely be sold even if we stay up (Vicario), and others will be a case of a transfer happening because it suited all parties, like most transfers you see.
