Have a read of this from someone else's build, gives a bit of info about buying steel & should give you a better idea what you should be paying. Company you mentioned are probably ok if you want just a small length but are expensive if your buying full lengths, the 6 mtr limit will probably have something to do with maximum length they can get shipped. Most steel stockholders will only sell in full lengths but should be able to cut for you.

http://www.mycncuk.com/forums/router...machine-4.html