You can get to Roissy airport by using a parisian taxi by car or even better ! You can choose a « moto taxi » in order to avoid the traditional traffic jams in Paris.
Parisian taxi prices vary of course depending on the distance travelled but also depending on when you take the taxi ( bank holidays, night time etc...).
Generally, it costs about 50 euros from Paris to Roissy using a « normal » taxi. The price by motorbike is a little more expensive, for example around 95 euros for a run between Roissy/La Défense by "taxi moto" .
The journey time from Paris can of course vary depending on where you are and if you take a « car taxi » or a moto taxi ». The journey usually takes between 30 mins and 1hour to get from Paris to Roissy CDG.
Be careful at certain times of the day, allow plenty of extra time if you are taking a taxi during peak hour traffic.
Webcab offers a shared or private taxi service at a unique price for Roissy Charles de gaulle. You can take Webcab alone or in a group at a discount rate of 40% compared to normal taxi prices. See the Webcab website for further information.
Finally, there are private shuttles which offer an easy, economical service to and from Roissy Charles de Gaulle. These shuttles offer transfers, on reservation ,between Roissy and Paris.