Prebuilt systems only LOOK like they're less expensive, because mostly they use inferior products. Most of them use cases that are cheaply made, and many of them won't accept replacement hardware later on because the designs are mostly proprietary. They usually include VERY cheap power supplies that in many cases won't live through the first year and often barely support the hardware much less any upgrades you might make later.
The support for most of them is abysmal, and for most plans, will cost you more money if you need to send it back because something goes wrong.
There ARE some prebuilt systems out there that are done well, but of the ones that are, none of them are less expensive than doing it yourself and in EVERY case where a prebuilt costs the same as a DIY system, the hardware will be inferior.
Is there anything you have already, that you DON'T need to be included in the build?
What country are you in?
What is the primary usage of the system?
Are there any "special" considerations?
What prebuit have you been looking at?