if you're building a really cheap priced system, you want to go with a manufacturer that does automatic parts for multiple systems, they are generally called "cloning companies."
however, I will guarantee that unless you are getting a very (I mean very and cannot excessively use words beyond VERY) good discount on a higher priced computer, you are not usually going to find a better deal than a computer you could put together yourself.
the reason why, is because more people would settle for a cheaper priced computer than spend the extra amounts of money on a solid monster machine. then again, consumer who don't have that great of computer knowledge prey on mid range systems with mediocre system performance and usually come with very "weak" graphics cards, yet sleeker looking cases. and then the high priced systems, supposedly to carry the latest computer technology, are made by companies like ABS, Falcon Northwest, Alienware, Monarchcomputers, Sagernotebooks, etc. - they will put on a heavier price tag. Often times you are getting less for than what you pay for, but guys like Sagernotebooks might be different, since they specialize in high end laptops, price from $900 to over $3,000, and mobility is one thing that desktops will never achieve in our current day, the ability to carry with you a system to serve your mobility needs without the extra bulkiness and messy wires.
I am confident that most enthusiasts here could build a faster system and far more reliable desktop system than what Alienware could offer right now (provided money and seemless effort). All you have to do, is take their parts from a highend gaming rig, and calculate in price what you get when you buy your own hardware and software to put together to make have a rig of your own custom-built savvy needs. Basically, you're paying Alienware to assemble and test for you a system they have agreed on their own standards to play the latest games. Not to mention, order parts for the system, preload it with software, provide tech support and all of that stuff you know a company will need in order to be successful and sell to their target market groups, gamers, workstation users, common (budget) user, etc.
You gotta know your math when it comes to technology and base it on feelings, like how fast you want it to run and how much money you have available, how long you've decided to wait, how much longer you decide to search for, etc.
My old math teacher once said:
"You need to know math, or else you're going to get ripped off."