McCarthy arrived in the summer from Wigan Athletic.
"The investment we made in James was a big investment but it was a safe investment because I knew his character inside out," said Martinez. "Everyone can find out about the player by watching the games but when you get to work with a character like him, you know it is a safe investment.
"That is very important for us because on the character side of things the players at Everton have to be perfect. And I think we have a group now who respect each other, they know each other and they have real high standards and that is why it was a real safe bet at whichever price.
"The figure is what you are prepared to pay and how much other teams rate him and how much you can increase that value in the market. The value of James McCarthy in the market is probably double that figure now if not more."
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