I can imagine it's costing quite a bit. However, traditional advertising is also expensive. The promo got me to order some stuff for the first time and in the process I saw that they do have good prices on some things. As a result, I will check them again for comparison when I need parts.
However, they really need an overhaul of their online shopping experience. Browsing for stuff takes way too long. Unless you already know the part numbers of what you want, some things are nearly impossible to find and any useful specs are buried on individual product pages. It simply requires too much clicking to find what you want and then manipulating search results for things like sort order can completely lose the filtering that was in place. I'm actually surprised I was able to complete the process in time to get a Pi.
I'd rate their site 1.5 out of 5 stars for shopping functionality.