I doubt that HD is taking a loss on anything, but with their purchasing volume with General Cable, they probably buy millions of feet per month. I'm sure that this, combined with buying directly from the manufacturer, they get it at a much lower cost than many supply houses. Same thing with Lowe's and Southwire. Even the bigger (national) supply houses probably don't purchase with that kind of volume.
I do find that staple items, such as cable and plastic boxes are cheaper at the home centers than at supply houses. Both HD and Lowe's have commercial sales desks and will offer certain discounts based upon your monthly purchasing volume. You need to ask them about it. I have a commercial account with both and I think that I get slightly better pricing than the advertised prices. Honestly, I haven't checked lately.
Grainger is great for those hard-to-find items, but I'd never buy any kind of electrical supplies from them unless it's a 600 volt pin/sleeve connector or something like that. With them, you are paying a premium for the convenience. I haven't bought anything from them in years, but they sure do come in handy when you are looking for that strange reversing starter. I must say that their inventory is mighty impressive to say the least. I don't mind paying more for that kind of stuff.
To answer your question, I really don't think that you are being ripped off. Discounts revolve around purchasing volume, and like you said yourself, your purchasing is relatively low. I know it's a Catch 22 situation, if you could get better pricing, you could do more volume, but the suppliers aren't going to take the risk. They expect you to make the first move.
Don't forget that the big box stores are very competitive with each other. If Lowe's has Romex at a price less than HD, HD will match and sometimes even beat it. Again, it's a matter of asking the question. Hope this offers you some insight.
