Paris - Oslo with Ryanair, round trip : 30€ (shuttle to the french airport) + 120€ (flight) + 68€ (shuttle to the norwegian airport) = 218€
Paris - Oslo with another company, round trip : 0€ (the Parisian airport can be reached without costs for Parisians) + 136€ (flight) + 15€ (shuttle to the norwegian airport) = 166€
Then the smashers from Lille objected that it was only if you were leaving from Paris, and that there were cheaper flights from other Ryanair airports. That's very true, but at best, the Ryanair trip would only cost about 10€ less.
Also, the whole shuttle thing would also take 3h round trip in France, and 4h in Norway, while the regular airports probably don't take more than 2h for both, round trip.
So in my opinion, if a trip is a bit more expensive, but saves a decent amount of time, and allows you to travel in better conditions, it's definitely worth, unless you can't afford to pay a few euros more than the lowest price you can find, in which case I think you shouldn't travel.