The through fare from central Paris to Orly via the Orlyval you mention is €12.05 (vs €3.55 for the equivalent journey to zone 3).
I guess your other option from where you are (assuming you're near Cour St Émilion station) is to take M14 to Bibliotheque FM and change to RER C as far as Pont de Rungis where there's a shuttle bus to the airport. This RER runs every 15 minutes and takes 22 minutes, with a connecting shuttle bus taking 5-10 minutes. Through ticket is €6.25.
[oops, misread - assume directions in reverse!]