£40m rising to £45m with add-ons, so I'd guess about £44m paid to date (including agent fees / transfer levy).
I'd be surprised if his contract was only 3.5 years, at least 4.5 more likely.
On the basis of 4.5 years, he'd be on the books at just shy of £30m right now.
But seem to be rumours that there'll be no deal and if the Minteh deal goes through then as
@Hyena says, they probably won't need to sell Gordon as well. But I stress "probably" because it's hard to tell if that will be enough to get them over the line. They might need to do both.
But Newcastle should know to a good level of accuracy how much they need to raise. Nothing happens, financially speaking, in June, so they should know what their numbers are going to look like,