Not cheap at all. I enquired about the Avon 22L bladder about a year ago and was quoted something over $ 900 CDN, which back then would've been about 450 British Pounds.
The value of our dollar has increased considerably since then (at least relative to the US dollar).
I've gone this route since it is about 1/10 of the cost, more rugged, and is also a safe way of transporting fuel.
Using Jerry Cans