Upvote:3
If you hail an "official" taxi in front of the airport you can expect between 40 and 55 EUR for a drive from Orly to Paris intra muros plus 15% for a drive in the evening. Denfert-Rochereau will be on the lower bound of the quoted prices. Pasteur a bit further away.
I found for the date you specified a quote from a taxi company, between 26 and 31 EUR from Orly Sud to Denfert-Rochereau and 34 to 42 EUR to the Pasteur Metro Station.
You can also hire a limousine service, a so-called "VTC" (voiture de tourisme avec chauffeur), you will have between 35 and 40 EUR from Orly to Paris intra muros (whatever the exact address). VTC services are totally legal in France.
The 20 EUR that have been mentioned by the other person apply to the controversial UberPOP service.