Just a personal observation - the obvious bike protection purpose aside, for the obvious purpose of taking your bike on the trains here I think you can get away with just about anything that covers the bike. We did a one-way ride last year and the couple of people who didn't have bike bags wrapped their bikes in gomi bags and duct tape. No problems on the trains.
That said, I'm pretty happy with my bike bag from the local Giant store. Cost about ¥7000 so a bit pricey.