I think, i don't suggest you by the taxi from CDG to Gard de l'est because I know they can see you are tourest... Normally is not so far, not more than 35€ because is so early morning not may car on the street but if you, maybe more than 75€. The best way take RER B after take the line 4. is about 11€ per personne. If you will stay in Paris, you can buy the ticket is for one day, 1 week, 10 days and 1 month. But when you must buy 1 ticket is from CDG to Paris first. After, if you will stay in Paris 1 week, you can buy for 1 week and depend on you want to go Zone 1, 2,3,4,5??? for ex: If you will stay in Paris 1 week and just zone1 and 2 not go to zone3, you can just buy the ticket is zone1 and zone2. Because if you will go a lot of place, buy the ticket 1 day is you can use this ticket at the end of the day. you can check ratp.fr