But you're missing a critical point here. Every player that has gone, has wanted to go, and to a greater or lesser extent, asked to go.
You only think Mane was a bad price because of what has happened afterwards. The majority of Liverpool fans thought they'd WAY overspent on him at the time (and most people here thought it was a reasonable deal). It only doesn't stack up when you see the likes of Boalsie going for a lot of money, but I think perspective is also a bit clouded because he's started so brilliantly. Half of last season he was poor for us. Let's wait and see. I think the £34m is "about right" - it's not like we've been ripped off, at this stage of his development any how.
I don't think we just see a figure and automatically say yes. If Palace came along and bid £30m for Hojbjerg, I think we'd turn them down, but even if we didn't (because we promised a player that we'd listen to bids if they gave us another season) PEH is not suddenly going to say "Oh, I've always dreamt of playing for Alan Pardew and Selhurst Park is my theatre" - no matter if they're in Europe (remember, we are too). The clubs we've sold our big players to have been Arsenal, Liverpool, Manchester United and Tottenham. All clubs that are waaaaaay bigger than us and can offer a lot more. We'll continue to sell at relatively inflated prices to them as it's otherwise a battle that's too hard to fight. I think you're getting clouded by the idea we sold Mane cheaply.