Van Persie was £22m.
Kroos was £20m in 2014.
Gundogan went for £21m last summer, fair enough injuries may have taken off 5-10m.
Mkhitaryan went for £27m after a season when he scored 23 goals and got 32 assists.
Hummels went for £30m - and he was going to a direct rival who the clubs can't stand.
Those are the ones which I can remember, if there's other top players who moved with a year to go they might add context but I doubt they'd be anywhere near £40m.
With Arsenal's only other choice being selling him to Chelsea, a move which their fans and Wenger would absolutely despise, £35m would be reasonable.