I think thats a very good point, and personally, i would rather take the time inspecting and cleaning/sanding earth connections (and possibly replacing any damaged or poor cables) than spend money on a kit that may even have inferior cables and lugs in the kit.
$150 is very easily spent on cables and lugs. Good lugs (insulated or uninsulated) are pricey, same as cables. gauge or area of copper does not reflect quality of metal or manufacturing.
My opinion: if someone is serious about it, get access to a good multimeter or ohmeter (or even a ductor would be best) and measure overall resistance from certain earthing points, all the way to the battery negative (disconnected of course). The biggest weak spot is the connections (surface area, surface material, lug quality and the way it is crimped onto the cable), cable size should have a negligible effect on resistance it is purely for current-carrying capability.
As far as acceptable resistance for a 12V DC system... i cant really give a figure. But say if you measure a few paths and they're <2 ohms, then you find one that is 4-5 ohms, i would look for the weak spot.