My experience (as others have stated) is that it's cheaper to buy a complete bike than to build, especially if you take in the cost of the tools you will need and if you put a cost to your time spent assembling the bike.
I'm typically looking for deals on last years model. If your patient you can find some good deals. The last bike I got was full carbon, disc, ultegra, crappy aluminum wheels and generic stock seat/cockpit/tires for a few dollars more than a full ultegra groupset.
The money I saved on buying the bike went right back into a new cockpit, saddle, wheels, tires, bottle cages, etc. I ended up tearing the whole bike apart anyways just to make sure it was greased and assembled correctly (glad I did).
Unless you have a dream build in mind that doesnt come already assembled, taking the pre built bike is the way to go.