That would assume we're 100% certain to make the playoffs or course. 538 have us down at 13% chance of promotion: https://projects.fivethirtyeight.com/soccer-predictions/championship/
I think they are perhaps closer to the truth, but let's split the difference between them and you and call it a 19% chance.
19% chance of getting £100m for going up, or 100% chance of whatever money comes in as a bid for Dickie. If it's £20m, at this stage, perhaps it's therefore not worth accepting. £30m and then I'd say the 'common sense / business sense' decision would be to sell him.
Football isn't all about the accounting though, much as it might feel otherwise sometimes, and so the board may decide to go with hearts over heads and reject even a very large bid.
They would I hope, if they have choice, of viewing what they maybe offered now against what he maybe worth if he has another good season with us.