The Welsh winger has been linked with a move back to the Premier League, and Chelsea and Manchester United are thought to be interested in signing him.
Bale has come under strong criticism this season from the Madrid press, while he has yet to win over the fans.
“If [Bale] does become available, if Chelsea win that battle and let’s say they lose Oscar, or Ramires, or a player like that for £50-60million, and they get [Bale] for a net cost of £25m, I think the rest of the teams are in trouble for the next couple of years, I really do,” Neville said on Monday Night Football.