Best Deal:
For zones 1 & 2 inside Paris central, excluding access to any Airport, each single one way trip costs €2.10 using an unrestricted t+🎫ticket. These are time-unlimited & date-independent tickets.
Depending on the number of journeys, it maybe cheaper to buy a carnet of 10 tickets 🎟️ rather than multiple individual t+ tickets. Normally, a carnet of 10 tickets 🟰 €19.10. But if you buy first a new Navigo Easy card costing €2 as a one-off fee, then load it with ten t+ tickets for the price of €16.90. Thus the total cost of these ten journeys now 🟰 €18.90. Hence, the cost of each journey now 🟰 €1.89 per trip the first time or €1.69 for every trip in the future.
If you are taking 5 or more single journeys per day, it would be cheaper instead to buy a Mobilis ticket on my Navigo Découverte card costing €8,45 per day offering an unlimited number of journeys per day. The specific Mobilis ticket is date-dependant and can be purchased in advance for any given day and spent when valid; except that...
The Navigo Découverte card can only contain a maximum of 2 Mobilis daily tickets at any one time; so you might have to wait for a day first and consume one Mobilis ticket ☝️ before purchasing the next one!
If my visit is for longer than 3.5 days, it would be cheaper instead to buy a Navigo weekly ticket costing €30 and add it to my existing Navigo Découverte card for an unlimited number of journeys per week and valid for all 5 zones {including the airports}. However this option is only valid for a predefined window period running from Mondays 00:00 till Sundays 24:00 & can never be purchased on or after Thursdays.
Source = ParisByTrain.
