There are many options for getting to Paris. While your first inclination might be
to take a taxi, there are a lot of more economical options at your fingertips.
[Please note that mention of a service should not be considered an endorsement
of that service nor a price guarantee of any sort.]
From Charles de Gaulle airport to the center of Paris, a taxi will cost you 35€-45€
by day and 45€-50€ by night. Orly is about 15€-20€ less. According to the terminal
used, the prices can vary by up to 6€. Some taxis can be reserved online. Do note, however, that some other taxi services are listed for as much as 60€-85€, so caveat emptor.
There is an "Airport Shuttle" which you can reserve online. The price appears to be 55€ for groups of 1-4. This can be particularly useful if you're in a group and you're planning to bring a lot of luggage, which I don't particularly recommend anyway. The shuttle only appears to serve Charles de Gaulle.
Then, of course, there are even cheaper options such as Roissy Bus (7€) and the Air France Bus (9€-12€), but both of these only stop at certain locations in Paris, so you'll still need to take a cab or other transportation option upon arrival.
The RER Roissy Rail train provides a cheap option for getting into Paris at about 8€. It's relatively easy to transfer to Metro at Gare du Nord or Chatelet, depending on the amount of luggage you have, and of course Metro is rather flexible, if you don't mind having to walk the last bit. Trains are a very inexpensive option for the budget traveller with a backpack or a sturdy wheeled suitcase.