Arriving CDG on a Thursday afternoon and leaving via CDG on Sunday evening of the same week. I figured it would be better to get a weekly Navigo at 21.25+5=26.25EUR instead of paying 10+10=20EUR just to get to/from CDG (booked a hotel in the city). We have plans of visiting Versailles too, and although I can't find the actual price, it's probably roughly 8EUR round-trip. Plus of course, we also intend to tour the city. Just need to know if we can still get a Navigo for the week we arrive :)
Thursday before end of service you can purchase Navigo Decouverte for the current week which will cover your travel up until Sunday end of service.
I agree that for most people the best transport pass to purchase is the Navigo even on Thursday since it now works in all zones, covering airport travel, Versailles, Disneyland, Fontainebleau, etc.