If you can afford the accomodation and flights / train I'd sort them first then worst case scenario you can always buy a return or go to a tour on the night of the gig.

If you buy a ticket 1st you're pretty much stuck if the accomodation and travel is too expensive later on.

BTW paris is MAD expensive for food, drink, accomodation and just about anythng you can spend money on just so you know...