The best "arrondissements" (districts) to stay are in order 4 (Bastille best location but I am biaised I was born there),3,5,6,9,2,1, the cheapest are 11,20. Hotels prices increased but they are still affordable. for less than $150/night you should be able to find a decent place.
CDG, the town, avoid cabs at any cost, it is outreageous.
You can take the RoissyBus it will bring you to Opera for less thant 10 euros, if you do not have too many luggage you can take the RER which goes non stop to the center of Paris.
They sell a museum pass which combine the use of the RER and free access to museum, if you put RATP (which is the public transportation web site) on google it will bring you the site where you can find some informations
I do not think the Champs elysees are interesting but it is because I am from Paris.
If you want to see some really local area go to the 20th around the Pere Lachaise.3 days is not a lot, I would go to Le Marais (3,4) and quartier Latin (5,6)
Other tip if you order a drink in a cafe the price is regulated at the counter, when seated at a table it is more, prices are always shown.
Have a good trip