Nothing. I just started pricing components and stopped there when I realized it was going to be $100 here and $100 there when I can get an entry level kit for around $600 (cheap china ones). Add on to that that you will need to make your own custom machined parts, source all your gears, bearings, slides, etc... and more than likely purchase a ready-made extruder and it seems like a lot of money and a lot of effort.
I wanted to see what it would take or if I should just throw that money in a jar to save up for a kit. The kit is winning out so far.
BTW, the easydrivers can only source 750mA/phase. Keep that in mind when looking for matching steppers. Most 3D printers seem to based around Allegro A4982 based drivers such as the stepstick which I see for around $10 a piece on eBay.
However, there is the BigEasyDriver which is capable of 2A/phase and 16 microstepping. The bought doesn't really look like it will handle heat well, though.
Now, I would definitely feel much more comfortable building my own after owning one and experimenting enough to know how it works, what the limitations are, what I like and don't like, etc... But for me, at least, it looked like a lot of money with a lot of frustration, and no guarantee that it will work as good as an established kit.
YMMV, of course.