Paris has 3 airports (Orly, Roissy, Beauvais), what are the options to go to or to come from there?
(There is also Vatry (XCR) but there is very few flights and i see the shuttle system is currently off.)
For more details (time, maps) about the 2 first airports please check these links:
RATP - Access to airports (English version, some links are dead for the cheap ways, it's a plot!)
RATP - Accès aux aéroports (French version, no dead links.)
About the tickets t+: I put the price of the tickets if you bought a pack of 10 (13.30 €) at a machine. If you buy only one at a machine, it's 1.70 €. If you buy it from the bus driver, it's 2 €.
Roissybus costs 10 € and goes to Opera.
RER B costs 9.50 € and can go anywhere in Paris, because you can use the same ticket after to take the metro.
There is a free metro system inside the airport called the CDGVal.
During the night, N140 and N143 go both to Gare de l'Est (4 tickets t+ (5.30 €)).
Cheap way: During the day, bus 350 goes to Gare de l'Est and bus 351 goes to Nation (3 tickets t+ (4 €)).
Orlybus go at Denfert-Rochereau and costs 7.20 €. (A few other stops are available, not the same depending if you are coming from or going to the airport.)
Orlyval is a metro between Antony (RER B) and Orly airport. It costs 8.70 € for only the way between Antony and Orly (you must pay the RER B Paris to Antony, 2.60 €), or you can have the whole ticket Paris - Orly for 11.30 €.
With RER C, stop at Pont de Rungis (4.10 €) and take the bus NPPT "Paris par le train" to the airport (1 ticket t+) (the whole way can be bought at 6.60 €).
During the night, N31 and N131 to Gare de Lyon (3 tickets t+ (4 €)).
Cheap way: During the day, bus 183 to Porte de Choisy or bus 285 to métro Louis-Aragon (1 ticket t+). You need a new ticket to take the metro after.
Beauvais-Tillé airport is the low-cost airport, 90 km away from Paris, used mainly by Ryanair and Wizz air.
The main option is the bus from Porte Maillot, it costs 17 € and takes around 1 h 15 depending on traffic.
After you landed in Beauvais, you just have to go to the bus (follow everybody) which should leave 20 min after. (If the flight is late, the bus will wait for it.)
If you have to take a flight, it is recommended you take the bus at Porte Maillot 3 h before your flight departure.
Cheap way: Low cost has its price, so there is not much a cheapest way. You can take the train from Gare du Nord to Beauvais, then bus (1.20 or 4 €) or walk 5 km to the airport (it's easy, but still prepare a map).
For an adult the train is 13.80 €, this way is only worth if you are 12-25 years old so it's 10.40 € depending on the hour (or if you have a SNCF card).
For transferring between these airports, the quickest option is the Air France coach ("Les Cars Air France"). You need line 3 ("ligne 3"). It departs twice an hour, takes around 1h30. One-way tickets costs €19 online or €20 onboard. It's advisable to buy from the bus driver than wait in line at a koisk/machine. The driver will load and unload your luggage.
Alternatively, you can schlep your bags on the RER B train. This is better if you want to stop off in the city centre. Take the RER B train from one of the two stations at CDG and change at the Antony RER station south of Paris, then transfer to the ORLYVAL shuttle or train to Orly airport. Alternatively, change at Denfert-Rochereau to the Orlybus. Its strongly recommended not to get off at Chatelet-les-Halles, one of the biggest transit hub in Europe. Very little of the journey is above-ground and is not particularly scenic. (A single ticket costs 20.30 EUR (cf. ratp.fr).
Another option is to take the Cars Air France. This is a private company that operates coaches to serve the airports.
There are 4 lines: