A while back I posted a photo of a home made camera bag. Its a basic rucksack £9.99 with a few modifications. It will hold a camera plus lens Canon 100-400 lens or my 170-500 with a few extras.
http://www.worldphotographyforum.com...3&d=1139607442
What is useful as well it doesn't have any of the camera bag names on it so it it just looks like an ordinary bag.