The Spaniard says no amount of money would tempt him into selling the Slovakia international and says he is grateful for the strength of support shown to him by the club's board
The 26-year-old midfielder attracted interest from Europe's biggest clubs in recent months before signing a new deal in Naples that extended his stay at the San Paolo until 2018.
And Benitez insists that no amount of money would tempt him into letting the Slovakia international leave the Partenopei.
"What is Hamsik worth? Is Bale worth what Real Madrid paid for him? In this market if you say an outlandish figure then a club comes along and pays it," Benitez told Sportmediaset.
"That is why I say that Hamsik is priceless - because we do not want to sell.”
Hamsik has made a fine start to the new season, scoring four goals in his opening two Serie A games, and Benitez also took time to thank the club for the faith the board have shown in him during the transfer window.
Following the sale of Edinson Cavani to Paris Saint-Germain for €64m (£53.8m), Benitez has been able to bolster his squad with the signings of Real Madrid trio Raul Albiol, Jose Callejon and Gonzalo Higuain, PSV winger Dries Mertens, and Liverpool goalkeeper Pepe Reina.
"Five names have replaced [Edinson] Cavani," added the former Liverpool and Valencia boss.
"I appreciate the confidence De Laurentiis has shown in me and [sporting director Riccardo] Bigon has done so much to improve this Napoli squad."