Agree with what you're saying, but if we get a young(ish) player (around 24/25) for a 'lower' fee because of his contract situation but pay silly money in wages (bringing an equilibrium to what we'd have to pay if he had 2-3 years left on a contract) you're also then left in a situation in 3-4 years time when that player comes to renew and they want even more money again to re-sign. In the longer term I don't think paying stupid wages is cost effective. I understand the situation with Sanchez is different because this is likely to be his last big contract and he's trying to maximise his salary in the short-term.
I'm sure others have probably made the point already, but I'd imagine a number of players will start to run down their contracts as they know that the money the club saves on transfer fees inevitably works itself back to the player. Hopefully City can avoid this by nailing down some of their younger players in long term contracts now and review them every 2.5 years or so to avoid a Sanchez/Ozil situation.
As for Sanchez himself, I'm not that arsed. He's been awful this year and clearly has a giant ego that constantly needs massaging. It's clear half the Arsenal squad dislike him and will be glad to see the back of him. I don't think we need a character like that in the changing room, regardless of how good he might decide he wants to be when he's happy. He'd have obviously improved the team, but at a cost both financially and to the atmosphere of the squad.