The best folding bike bag I ever bought was that Dahon Stow Bag. I think I paid £60 for it back in 2010.
Tern sell basically the same bag but it ain't cheap. I think it costs about £120 but it really is a very solid bag so might be worth the money I guess.
These days I use a bag I bought in
Halfords that just about does the job, although one of the zip pulls did break after a few months.
I would use my Dahon bag but I left it at a relative's house overseas along with my Speed D7 when I went to visit them 8 years ago and haven't been back since.