The Serie A giants face Barca on Wednesday in a friendly as they continue their build up to the 2014-15 season and Benitez claims his team cannot stand toe-to-toe with Luis Enrique's men.
The Catalans shelled out £70m to buy Luis Suarez from Liverpool this summer, with the Spaniard pointing to the economic gulf between the sides.
"In my opinion, there is a difference in principle with regards to the expenditure of the two teams," he told reporters. "I think it's about €500m.
"Napoli have a strong balance sheet and it will certainly improve with time, but today our financial potential is miles behind that of Barcelona. For this reason, mainly, perhaps the gap between the two teams remains remarkable.
"It will be a prestigious challenge against a team which have plenty of quality."
The Spaniard was also non-committal when asked whether Gonzalo Higuain will feature against the club he was linked with earlier in the summer.
Benitez said the striker "is eager to return but he is not yet at 100 per cent" after Argentina finished as runners up at World Cup 2014.