In reply to AymanC:
> I really should have just booked an extra seat on the plane!
Seriously, If your mate can't afford the trip you should just try and find someone at your destination to climb with rather than taking him as luggage......
I have used a 100l (they go up to 140l I believe) mountain equipment duffle for years that was quite cheap and managed to carry that along with a large pack through airports many times. If you go too big you'll likely exceed the single item weight limit. I think the longest side is always 75cm on the larger ones though. They come in roller versions too but I always got by using a shoulder strap on each pack.