Hi Fred
Dont know how big the layout is or the amperage drawn. I purchased 2, 250 foot rolls of 12-2 romex at about $75 per roll last year. When I priced it at Lowes it was the cheapest per foot,cheaper than 12-3 or 10-2 as far as copper per dollar. For less demanding loads, 14-2 is just as economical. I think the reason is they just produce a lot of those 2 runs,since it is the most commonly used for house wiring.I also used some 10-2 that I had left over.
Funny thing was too. 250 foot of 12/2 was about $75. 100 foot of 12-2 was almost $50. The 1000 foot roll was actually more money per foot. So these outfits all play games with prices. Checked Home Depot and prices were the same.
Fairly easy to strip the individual wires and pull out of the jacket to use individually. Using the grounding bars easy to make connections to. I placed them about every 10 feet around the layout perimeter in a loop. So I have a common return with almost no resistance. Saves a lot of wire. Not recommended for DCS though.
Anyway we are looking at a low voltage return system here so I do not see much danger. Everything is properly fused, and the amperage drawn is not much more than an automobile,where the chassis is used for a common ground return. Never heard of anyone getting electrocuted off such a system or touching the car body.
Dale H