cheap vs expensive flexplate

You have to DEFINE the term "balanced."

360 LAs are externally balanced by the balancer and weighted torque converter, and THEY are different for a Magnum. My understanding was that the Magnums used a special flexplate as opposed to a weighted converter.

The first link you posted at rock auto is for an INTERNALLY balanced engine like a 318, OR an externally balanced 360LA WITH a weighted converter

The second link you posted, the Summit / BM, is an UNbalanced flex plate which allows the use of a 360 LA (in this case) with a neutral balance, unweighted torque converter.

The photo Summit posted is "representative", IE not correct. The BOTTOM of the three in the photo most closely resembles what you are ordering there, IE the cut out area takes the place of the torque converter weights. NOTICE that fits only a 904, not a 727