His value depends on two factors, the length of his contract and how many clubs are interested in buying him.
Arsenal got a much better deal than they were expecting when selling Sanchez simply because both City and United wanted him. Once two or more clubs are bidding then the length of the player’s existing contract is largely irrelevant. The competition between City and United drove up his price until City decided that it wasn’t worth topping United’s most recent offer. We made that decision based on his age and perceived value to us, it had nothing to do with the length of his contract with Arsenal.
From the outside it seems that Sane has set his sights on moving to Bayern, and only Bayern. If that’s true then we should sell him when we think that we can get the best offer from Bayern.
But has he categorically ruled out moving to any other club? If not then I agree the ideal situation would be for him to play a few games in the hope that stimulates other interest. The golden scenario is that he plays a few games for City, does well enough to make the Germany squad for the Euro’s and then a fully fit and fresh Sane stars in that competition. A successful high profile summer tournament is a sure way of increasing a player’s value.