Those little unforeseen costs will bite you hard. Got a 1 1/8" drill bit? *Ding* there's $8. Oh, what's that? you want to be able to open your CP. *Ding* piano hinge - $15. You plan to paint it? *Ding* $20. Oh, you're gonna laminate it instead since it looks so nice and the laminate is only about twice as expensive as paint. Wrong, cos now you need a bucket of contact cement, throwaway brushes for applying it, a J-roller and some a bunch of dowels, not to mention a flush trim bit for your router. *Ding* add another $60 to the cost of the laminate. You're installing t-molding, right? Wrong! At least, not without a slot-cutter bit for your router -- $20. Want a keyboard tray? Drawer slides will run another $15.
Screws, glass clips, hinges, wires, casters, glue, clamps, files, chisels, drill bits, sand paper. And this is mostly just for supplies. You'll likely end up buying yourself a couple of tools if you don't have them already. Got a router? A jigsaw?
One thing that has got me loads of times is buying hardware that ends up not working. I run into a problem and think of a potential solution in my head. Then I wander around Home Depot or Ace Hardware looking for whatever I've imagined up in my head. I'll find something that looks like it will do the trick, only to get it home and test it out and find out that it's not going to work after all. Then it's back to the hardware store to spend even more money on another solution that *might* do the trick.