The value of Sane as an asset is judeged by City, not by you.
His previous two seasons have produced numbers similar, if not better to Eden Hazard, who has just moved for around €100m with only one year left on his contract and is considerably older than Sane in footballing terms.
No one really knows how much City are holding out for, but looking at the Hazard example above, it suggests that Sane should command a higher fee than Hazard, given that in both examples, only one club were interested in buying.