One "ticket t+" will cost you €1.80 and you can buy a "carnet" of 10 tickets for €14.10
If you're planning visits outside Paris, you can buy "Origine-Destination" tickets, depending on your journey. For example, from Gare de Lyon to Versailles - Rive Gauche - Château, one ticket will cost you €3.55
The price can be checked online at
http://www.ratp.fr/tarifs/
"Paris-Visite" can also be an option, as it allows unlimited travels on selected zones as well as journeys from/to airports (€22 for round-trip between Roissy-CDG and Opéra)
Hope it helps !
Cheers,
Florian