I haven't seen much variation in price retail - they are all around the £10 mark for the 600ml and £13 for the 900 ml. Field and Trek are as good as anyone on price but most camping shops stock them at about the same cost. Mail order seems to wipe out any price advantage with postage costs.
Alternatives - it's a bottle that you are going to fill with flammable liquid, pressurise a bit and put close to a flame - how cheap do you want it to be?