It's cost saving, but the cost saved is really weight (mass) of the fuel. Using the gravity assists means you can save on fuel and use that mass for the spacec

It's cost saving, but the cost saved is really weight (mass) of the fuel. Using the gravity assists means you can save on fuel and use that mass for the spacecraft and its instruments instead. JUICE masses 2400 kg, where Juno is 1600 and the Voyagers were 700.