If you want to get to Saint-Tropez faster, you will of course need to take the autobahn rather than along the coast through the cities. This autobahn is toll. You will need to pay the fare twice.
1-st payment point is located near Antibes (Péage d'Antibes), 15 km from Nice airport. For a passenger car you need to pay 3 euros 40 cents.
2-nd payment point is located at the exit from the autobahn, near the town of Le Muy (71 km from Nice). Here you will pay 5 euros 70 cents (for a passenger car).
Total for toll roads, one way from Nice to Saint-Tropez you will pay 3.40 + 5.70 = 9.10 euros.
The second item of travel expenses is the cost of fuel (gasoline or diesel). The distance from Nice to Saint-Pope is about 105 km. Fuel consumption will be approximately 10-12 liters, taking into account driving in traffic jams (also depends on driving style and engine size). You will spend approximately 12 * 2 = 24 euros on fuel.
Thus, the total cost of travel from Nice to Saint-Tropez (one way) is about 35 euros for a passenger car. By minibus it will cost more than 50-60 euros.