Use the Paris Transport Card
Transportation in Paris is extremely affordable, especially if you get the Paris Transport Card. The card will give you unlimited travel on the bus, metro, tramway, and RER networks and its suburbs.
Prices start at €13.20, and you can choose from 1,2,3, or 5 day passes.
Be sure to check the zones of your pass! Zones 1-3 are for the city center and nearby suburbs, whereas Zones 1-5 will include further away destinations like the Palace of Versailles and the airports.