Despite playing just 11 times in the Premier League this season, the 23-year old is attracting interest from other Premier League outfits but his signature would not come cheap says Karacan.
"Around 15 to £20million? You see how much young English players go for - there are some crazy sums out there," the midfielder said when quizzed on McCarthy.
"I don't see why he shouldn't be in that bracket. It would definitely take a lot of money to get him away from Reading - that's how our club works."
He added: "He's amazing - unbelievable.
"It's surprising for myself when you see the likes of Jack Butland getting called-up ahead of him.
"It would be a coup if we could keep him, but he loves it here, is playing here and that's all he ever wanted to do.
"You never know. He will get a lot of games next year, but there will be clubs knocking on the door and definitely a future number one."