The Wales international has been linked with a move away from White Hart Lane due to his prolific form in the Premier League, but Allardyce is adamant that the 23-year-old's value will continue to increase.
"The world of football outside of this country might decide his worth," Sky Sports News quotes Allardyce as saying. "There might be somebody else from another country that decides he could be the man, so that would inflate the price a little more. I would have thought that Tottenham would want nothing less than £40m-50m."
Bale has netted six goals in his last seven outings.