About price-shopping at Dell: I customized a Dimension 9100 a week or so ago to a particular configuration, and it came out as about $1350. I configured the Dimension XPS to something as close as possible to that, and it was about the same price.
Half a week-later, I repeated it, because the various deals had changed, and this time the XPS was over $1450, while the 9100 was about $1290, and came with a free printer (that I don't need).
Today I poked around the Small Business store. It used to be that Small Business charged sales tax, but Home didn't. However, Home does now in a lot of states, including CA, so no reason not to try it. And at Small Business, I built the same 9100 machine (near as possible), and it came out at $1000, with free printer, with 1GB of RAM instead of 512M.
It's possible there's some subtle difference I'm missing (you can compare machines on your wishlist within one of 'home' or 'small business', but not between both), but I think it's actually just the random vagaries of their random deals.