Wickham, 17, is the Tractor Boys' youngest-ever player and a target for the Reds, who are ready to swoop in January, as well as Premier League rivals Arsenal and Tottenham.
Keane said: "There aren't many Connor Wickhams around. You've got to be careful not to get blasé about Connor and say 'we'll take £7m or £8m for him'.
"I have a certain figure in my head that we'd have to consider. I think Connor and his agent would have that as well.
"The big teams wouldn't be buying him for what he's doing now. They're going to buy him for what he can do in four or five years' time - his potential.
"Whether that's Tottenham or Liverpool or whoever, but United had to buy Rooney for £28m when he was 18.
"Can Connor be as good as him? I think he has the potential, but that's up to the club. I don't think the club has had any concrete offers yet."