Box stores are usually competitive, sometimes less, and sometimes a lot less on most residential grade supplies and romex, even with the supply house contractor pricing. It's the other way around for the commercial grade. On top of that, the supply houses I use really do a good job of sticking it to the guy that comes in off the street. I've seen them charge someone without an account a lot more what I can buy the same thing for.
For example, I was in one afternoon when the counter salesman asked if I wanted to sell a dimmer to a lady that was there for around $25ish. $25ish was what it was at Home Depot was charging for it, but it was out of stock. I told him to just sell it to her as a cash sale, but run it through my account for the pricing. My price was about $20.00. After she thanked me and left, I asked what that was all about and he told me that her price was going to be around $40.00. The reason for that is that they want to take care of their real customers, and don't want someone coming in after I charge $40.00 for it and finding out that I bought it for $20.00.