Not very complicated, check their transfer history. Alves was the most expensive at £30 million and he was not only the premier defender at the time but a fullback which is a decidedly more valuable asset to a club like Barcelona than a central defender. Otherwise, Barcelona have never come close to those prices again for a defender, nevermind a CB. They came close with Thiago Silva but his wage demands were unreasonable but it makes the probability of a bid that high for Stones a virtual impossibility
He's far more likely to end up at another English club were he to leave Everton
Dani Alves joined what 7 years ago? If you use any football inflation calculator 30 million then is like 50 million now. That was also for a 25 year old not a 21/22 year old. I think it gives you an indication of the ball park figures people would be talking.
Ultimately money is no object. They look at their academy to fill positions with it often does and then use the copious amounts of money to go and buy special players they don't have in the academy. Even for Barca I would be surprised if they had another John Stones. He could well eclipse Pique if he continues his trend of development.
This season, partly through injury, partly down to Howard being a clown and partly down to his head being a bit all over the shop post the Chelsea stuff he hasn't been at his best. In some games he has still looked sublime though. Chelsea at home for example not only did he nullify Costa, but we built attacks through him. I think if he wants to play for Barca he will have to start showing his potential consistently, far more so than we've seen this year.
Make no mistake though, if he does, and the penny drops that he needs to lead that defence he will be a brilliant defender who will be perfectly suited to Barca.
You mention about Alves but the thing with him and Barca was that he was a bit ahead of the curve. A right back who played as a right forward. Stones has that feel about him too. Sides are increasingly tying full backs down. Defensive midfielders patrol infant of defences with crab like efficiency. Sides are well drilled to defend against it. Having a lad like Stones who can step out of defence and build attacks is a real game changer. When he played well we look a far better team and he gets the best out of Barkley in front of him. It sounds a bizarre thing to say, but then again if you rewound 20 years and said you'd have had fullbacks like Alves who's first job was to attack, would it have sounded equally bizarre?
At a side like Barca, where teams drop back and let Cb's have the ball and don't test them out as much defensively he would flourish. I remember Martinez saying about Deulofeu he has to get used to playing in team that doesn't have the ball for 75% of the game. Stones would be dynamite in a side that had the ball that often.
He has to up his game and consistency. He will know that. But if he does he would be perfectly suited to Barca, and your fans would love watching him.