Single tickets cost 2.50€. Carnets of 10 (now digital only) cost 19€. A Navigo Easy card with 10 rides is the cheapest option.
Buy the Navigo Easy at any station (2€ for the card + load with rides). Reload at machines.
For 1 week+, the Navigo Découverte (weekly) is 30€ and includes all zones — covers Versailles and the airports.