Benitez said that if Bale is worth the £85m that his new club paid Tottenham Hotspur for his services, then Hamsik is "priceless".
"Is Bale worth the £84m Real Madrid paid for him? In this market if you say an outlandish figure then a club comes along and pays it," Benitez told Mediaset.
"In that case, I say Hamsik is priceless and is not for sale, because we do not want to lose him."
Hamsik joined Napoli from Brescia in 2007.