Tottenham are the frontrunners to sign the Wales centre-back before Friday's transfer deadline.
But Cooper has warned the Swans won't sell below their asking price, believed to be around £18m.
"What I would say about Joe or any other of our assets – and Joe is right up there as one of our biggest – it's really important that if we do end up having to sell then we get something to show for it," Cooper said.
"We shouldn't be selling on the cheap. The precedent has been set this year."