Lowers range in prices for a few reasons:
Quality of Material
Type of build (forged vs milled billet) milled lowers take more time and material
Type of finish
Brand name (big reason)
Are they really worth it? probably not. I know some would argue otherwise, but a good majority of people have been buying spikes and psa lowers for <$100 for years with no issues. Others would rather spend the money to match their lower and upper, or to have the "quality name"
You cant go wrong with a more affordable lower as long as they are a reputable company (spikes,psa, etc)
As for the BCG, perhaps someone can chime in with the legality of using auto parts in a semi in the state of NJ. Everything ive ever read has said you can use it no problem, however im not sure if NJ puts a blanket NONO on any auto part or just the possession of all things to make a rifle full auto. For full auto, you also have to have the right safety, trigger, disconnector, hammer and bolt carrier