Tools was big for me. I basically had none. In the process of building mine, especially in the beginning I purchased a router, jigsaw, jigsaw blades, circular saw, staight edge, clamps, squares, tape measures, level, router bits, calipers, pens, pencils, dust masks, eye protection, drill bits, a mallet, chisels, wire strippers, a big tool box, saw horses, a file, a wet/dry vacuum, electric sander (which I'd gone orbital), utility knife, work light, extension cord, and other things which currently escape me. I'm easily $500+ into my project on tools alone.
Of course, the tools have been used in countless non-arcade-related situations since I got them, so it's probably not fair to count them explicitly as costs of my arcade cabinet. Also, I am more of a man since I got all those tools, so it was really quite a bargain.

But I'm well over $1000 on materials too (my PC was free). Off the top of my head I have (all purchased new):
Joysticks x 7
Mounting plate for Ms. Pac joystic x 1
Act Labs TV light guns x 2
Buttons x 35-40
Coin door x 1
Monitor bezel x 1
Marquee retainer x 1
Fluorescent marquee lamp x 1
Logitech MOMO Force wheel and pedals x 1
Act Labs shifter x 1
Cabinet handles x 2
Fans x 2
27" tube TV x 1
4'x8' 3/4" Melamine (prelaminated MDF) x 4
4'x8' 3/4" Plywood x 1
4'x8' 1/4" Masonite x 1
Laminate (a sheet of metal laminate was over $100 by itself!)
Replacement coin reject buttons x 2
3" Happ trackball x 1
Trackball mounting plate x1
2" generic PC trackball x 1
Aluminum t-molding x I don't remember -- enough to cover all my edges
7-port USB hub x 1
Quick disconnects x 200
Button wrench x 1
Smartstrip power strip x 1
And, of course, countless screws, bolts, lights, hinges, brackets, casters, wood glue, contact cement, epoxy, brushes, dowels, heat shrink tubing.
Good lord, listing it all out like that makes it seem pretty obscene. Thank god I got it all years ago when I didn't have a kid and wasn't in school and had expendable income. There's no way I could spend even a small fraction of that to build a cabinet today!
