Bale joined Real Madrid before the close of the transfer window in a world-record deal thought to be in the region of €100million (£85million).
Bale's price-tag has created debate around the footballing world and Benitez feels Slovakian ace Hamsik would be worth more than the Welshman if he was on the market.
"Is Bale worth the 100 million euros Real Madrid paid for him? If so, then I'd say you can't put a price on Hamsik - and he is not for sale," Benitez told Mediaset.
Benitez also hailed Napoli president Aurelio De Laurentiis for backing him in the transfer market to replace Edinson Cavani this summer.
President Aurelio De Laurentiis loves me and he showed faith in me," added Benitez. "He did his 100% to improve the team.
"After Cavani's departure I gave a list with five possible replacements and between them there was Higuain."