Mine is a £60 bag, with a free specialized cardboard box in it. It’s the box that’s doing all the work, the bag just gives it wheels, handles, and a bit of respectability and rain resistance.
If I didn’t have the bag and I had to travel with a bike tomorrow, I’d go to Halfords and pick up one of their boxes for an Apollo- reason being, they’re enormously tough- they have to restrain 50lbs of BSO with minimal packaging as it’s transported round the world for the lowest possible cost. And then I’d go to staples and buy a load of packing tape, and laminate the thing for tear and water resistance.