They aren't always that cheap.. I remember calling places, and the guy mentioned this special they had on.. It was Westburne Electric IIRC.. Regardless, I can't imagine paying more than 20-50% what they want at the hydro-store for the same unit in a nice white chassis, with fancy grommetted cords..
Just make sure there is a tap for your voltage.. Everything I've ever bought has been triple tap, but some might need 240V or even 377V (IIRC) only.. Not many contractors will be hooking these up to 120V, so its reasonable to assume that some might be made less costly by eliminating the extra taps..