Strange but true a lot of people swear by the big blue shopping bags you can buy in IKEA. This for Bromptons but they may be big enough for larger 20"ers. Failing that - think about laundry bags maybe?
Dedicated bike-bags are never cheap.
@ Dyno. Not wanting to stir but is that the best you can offer? It's like saying 'google it'. Not the most welcoming thing to say. Perhaps Arste has already persued these avenues, and this is why they're posting in a Folding bike forum - hoping not to get glib answers.