I have often marveled at how HD and other big box stores can sell certain materials at retail, below what I can get them for at my supposedly wholesale suppliers. They don’t sell at a loss, they just negotiate directly with manufactures and distributors for better volume pricing, and remove the middleman [suppliers] from the process. I don’t like that they sell these materials to non-trades people though, but now most supply houses do that too anyway.

Last summer when I needed to restock my supply of #12 stranded THHN, wire prices had gone way up since the last time I purchased this wire and I was blown away at the prices that all of the supply houses where quoting, which were from $99.00 to $129.00 per 500 FT spool of BK, RD, BL, WH, GRN.
I just happened to be at HD later that week for something else and saw the same brand of stranded #12 THHN for $60.00 per 500 Ft. spool.
Needless to say, at around $40.00 less per spool, I cleaned the shelf off.
I also noticed a significant price difference for 250 FT coils and 1000 FT spools of RX. Not so much for MC cable though.
You do have to be careful when you shop at HD though. Sometimes the quantities are different in the packages or on the spools and can actually cost more per piece or per foot than at the supply house.
Grainger is a great supply company for motors, but their other prices are definitely extremely high. No real discounts on tools either. I used to have to spend at least $5000.00 annually to get their contractor discount. Your also paying for all those pretty red and white boxes when you have items shipped