I lived in the Paris suburbs for 6 months (zone 5) but I always had a Carte Orange - now that I'm going back to visit, I'm confused about prices. Those cheap 't' tickets are only for central Paris right? So how do I figure out what a single ticket from central Paris to zone 5 would be? I've combed the RATP site and can't find it.
Thanks for the help!

I've never found a price list on ratp.fr, but I know they get €7.85 for a ticket to town from CDG in Zone 5.

I'm not sure it's on there (I have searched in vain to find various RER ticket prices), but it is 7.85 euro to CDG airport. I don't know if that is more expensive than the rest of zone 5, but I think it may be. I think I remember paying 35FF RT to zone 4 pre-euro days.

so if we're going from the airport into town and then back out again to zone 5 on a different line, would it be worth it to just get the Paris Visite for the day? (I think it's 16-something euros)

Yes. You have identified one of the few cases where a Paris Visite at €16.75 makes sense.

Can we get to the Orly airport with that, or will we have to pay extra?

Orlybus, Orlyval, and Orlyrail all accept Paris Visite for Zone 5. Here's the dope:

<b>http://www.ratp.info/orienter/aeroport_orlyval.php</b>