The 20-year-old winger has been linked with a step-up into the Premier League for some time, having impressed on a consistent basis in the Championship.
A big-money move in January has been widely touted for a man drafted into the latest senior England squad.
Browett insists Palace have no intention of allowing Zaha to leave, but has made it clear that were their hand to be forced, it would take a sizeable fee to land the youngster.
"If you finish last in the Premier League you get £60m and with parachute payments over four years you are guaranteed £100m," he said, with the Eagles pushing hard for promotion.
"Selling your best player makes no sense.
"The figure of £20m has been talked about and that has let interested clubs know we are serious."
Meanwhile, Ivory Coast coach Sabri Lamouchi has hinted that he will make another effort to bring Zaha into his squad for the 2013 Africa Cup of Nations, with the highly-rated playmaker still eligible to represent the country of his birth.
He said: "We've not decided on our squad yet but he is clearly very selectable. We have some way to go to convince him but I want to speak to him directly, and I know (Didier) Drogba has already spoken to him."