I’m going to be living in the 18th arrondissement for three months. What is the cost of an RER & Metro pass monthly? I used to live in Saint Germain En Laye years ago and the pass cost about $90. How much is it now?
The monthly travel pass is Navigo Mois. You load it onto a Navigo contactless card.
Because french people can't do simple things, there are many different Navigo cards (and they come up with a new one every 3 months these days). You can load a monthly pass only on a standard Navigo card or a Navigo Découverte Card.
The standard Navigo card is in theory reserved to people who live or work in the Paris region (Ile-de-France). You'll need to provide name, address, photo, and IIRC details of a bank account (what is called a "RIB", which shows your bank account number, etc.), even if you're not getting a subscription. It's free. If you lose it, you can get a replacement (including any passes loaded onto it) for 8 euros.
The Navigo Découverte is available to anyone, only requires a photo, but costs 5 euros and if you lose it, any passes loaded onto it are lost.
In RATP stations at least you don't need to bring a picture, they can take it directly at the desk. Not sure about SNCF stations (the RER station at CDG airport is SNCF).
Once you have the card, you can load daily, weekly and monthly passes onto it either at a desk or using the vending machines. You can even do it online, but you need to buy a card reader for that.
If you need to travel inside Paris (zone 1), there's no longer any choice for weekly and monthly passes, you can only buy a pass for all zones (1-5). The cost for a monthly pass is currently 75.20 € (22.80 € for a weekly pass).
Note that monthly passes run for a full calendar month (from the 1st of the month to the last day of the month), you can't get passes starting on arbitrary days. Likewise weekly passes only run Monday to Sunday.
The price varies depending on the zones covered.
From the Paris Convention and Visitors Bureau fares guide:
You can also choose a Monthly or weekly Navigo Package. These can be purchased from the ticket offices or machine in the stations, their prices vary depending on the travel zones covered (see "Passes and reductions"). You will need a passport-sized photo for these passes, obtainable from the photo booths in some metro stations (make sure you have the correct change).