Here's a place that claims to sell you the one day zone 1-5 Paris RATP pass, which will include the round trip to/from the airport as well as unlimited metro and even the roundtrip to Versailles, for $25US (plus $12 shipping!):

http://www.gotoparis.net/visite.html

Never tried buying it this way so I don't know how reliable this method is. Shipping method for this price is listed as "DHL ground" which doesn't seem very useful.

In theory you should be able to buy this pass at the Euro price from the RER ticket agent on arrival, if you can get past the traditional rudeness characteristic of virtually everyone who works at CDG.