We had to get to Paris Orly Sud airport by 4am for a 6am flight. The tourist information desk said we should take a cab for about 50 euros (we stayed in the 1er arrondissement), which we found very expensive. We enquired at the information desk in the metro whether any public transportation option existed to get to the airport by night, and they said to take a cab as well. However, we got a map of the night bus network (called the "Noctilien") and saw that there was a few bus lines that could take use to Orly Sud. We enquired (again) at the bus station to know how much we would pay, and they said just 1 regular ticket, but this was wrong.
We felt like they wanted to promote the cab industry and scare tourists out of the public transportation, but we did not let this stop us and we managed to get to the airport by bus.
If you want to get somewhere with the night bus, the best thing to do is to get your information online, mostly using the Noctilien website (http://www.noctilien.fr/Noctilien/pages/en/index.html).
With the Noctilien network map and website, we were able to see that there were a few different options to go there by night. We need to pay one ticket per zone (1 for zone 1/2, 1 for zone 3, and 1 for zone 4, thus three tickets per person). So we paid about 5 euros each (10 for two people) instead of 50 euros for a cab! The bus driver accepts cash and also gives change back, so you don't have to bother getting tickets ahead of time.
I highly recommend that you go find the exact location of your Noctilien bus stop the day before, because they can be a little difficult to find. For example, around the Châtelet metro station, there are about 8 different locations for the Noctilien, and you really don't want to be looking for yours in the middle of the night when you know the bus is coming soon.
In terms of walking and waiting outside at night, I guess you can see for yourself if you feel safe enough to do it. I was with my husband and the bus stop was in a pretty touristic location, so I felt OK walking about 15 minutes to get to the bus stop. There were police cars driving by the bus stop once in a while as we were waiting, and only one person talking to himself on the way there. So although I was a bit anxious at first about this unusual nightly walk, everything went well.
In short, no one will tell you in Paris, but you have other options than taking an expensive cab ride if your flight is very early in the morning. However, you will have to plan your route by yourself using the website, you will need to make sure you know exactly where your bus stop is beforehand, and you need to be ready to walk and wait outside at unusual hours. read more