Well, we just got 2 of FR's 1m 0p seats to get the parents-in-law over in December.
We had to pay for the return, so total cost for 2 pax including 1 suitcase was €152. This covers everything inc the card fee. BA on the same route would have been at least double price and the FR airport is actually nearer to their house than the BA airport.
That said, you also have to factor in a contribution to the FR emergency fund. Last year they cancelled the flight and we had to pay £300 to buy 2 one-ways on Easyjet for same day departure - a cost we would probably not have paid had they booked BA. However, they've done this flight about 15 times now and this was the only muck-up so far.