Keane turns 30 in January 2023. His contract runs until June 2025, so his book value is fairly low at just under £9.4m (£25m divided by 8 years = £3.125m per year). He's widely reported to be on more than £70k p/w.
Keane cannot play in a 3 CB system and would not, with everyone fit, be a starter in a 2 CB system. He cannot play elsewhere. He cannot lead/organise the defence. He ONLY looks any good in a back four playing a low block, with a big character alongside him to lead, organise and keep him from crumbling mentally.
If another CB on relatively big money comes in then Keane AND Mina will have to leave within six months otherwise our wages to income ratio will get hammered again, causing us more P&S issues.
I would take £10m for Keane right now. Absolutely no question. It would be a small profit against book value but it would take a hefty wage off the payroll and get a very limited player out of the squad.
If we wanted to make a similar small profit on Godfrey this summer we'd have to sell him at around £15m due to his current book value.
Holgate? Book value is super low because his original transfer fee was modest. Any sale now would be almost pure profit. There is a financial argument for selling him, yes. But he's on lower wages than Keane, is younger and faster, can play in 2 CB and 3 CB systems and can also fill in as a RB in a flat 4. He makes errors, but certainly no more than Keane.
Even if Keane was the better player (and he isn't), the numbers are pretty clear. An offer of £10m would be enough to let him go.