I've sent my bike in cardboard box quite a few times now with no problems. I wouldn't pay all that money for a bike box...
My brand new Bianchi which I paid £2,000 for in 1999 went on its first overseas trip in a cardboard bike box. Being a cautious type, I
also put pipe lagging on the frame and forks,
and wrapped it in bubble-wrap
and put the whole lot in a bike bag. I thought to myself
"That little lot should protect my bike from careless baggage handlers!"
So there I was, sitting on a plane at the airport, watching baggage being unloaded below my window. Suddenly I saw my bike bag come flying out of the hold and land on a pile of suitcases about 6 feet below...
I get the bag to my hotel, unpack the bike and discover that my new frame is dented. Just great...
I bought a SciCon bike box to use on subsequent holidays. There has been no more damage to my bikes, though (as mentioned above) the baggage handlers did manage to break the corner of the box on one holiday flight!