I highly doubt Barkley will go for 35m - he'll be in the last year of his contract come the summer, and if Everton get Davy Klaassen (as is being suggested by some outlets in the Netherlands), then they will have already secured his replacement, driving his price down further (since other clubs will make the link between Klaassen and Barkley). And if Barkley reaches the last six months of his deal in January 2018, there will be nothing barring him from seeing out his deal and leaving on a free (with the concurrent higher wages that come with joining another club on a Bosman), while Everton will get nothing because of him being older than the age necessary to claim compensation.
Given those factors, I'd wager 20-25m at most. The question is whether he's worth that, and I'm not sure he is.