There are always sales – see gotapex.com, slickdeals.net, techbargains.com, etc. Every so often there is a great deal, so it’s worth watching them for a while if you are not in a big hurry.
You certainly aren’t going to build a PC cheaper than what Dell will offer them for. It might be worth it if you like building PCs or you want features it would be hard to get on a Dell motherboard, etc. (Also, if you live in Texas, buying Dell isn’t quite as compelling… sales tax.)