The Premier League champions reportedly had a £12m bid for the England left-back rejected, but Ratcliffe claims that the Toffees could sell him for the right money.
"I would have thought the club would be looking for around the £15m mark if he was to leave," he told talkSPORT.
"I think if the right offer comes in Everton will take it and I think an improved bid of around £15m might sway things Manchester United's way.
"If Everton get the right money it may be right for both parties to part company, especially because of his age. It's unlikely they'll get this much money for him two or three years down the line. But that's only if the player wants to go."
Ratcliffe also said that losing Baines would be a bigger blow to Everton than the departure of midfielder Marouane Fellaini, who has also been linked with a move away from Goodison Park.