Hrm... good question. I never added it all up... lets see...

Cabinet
$350 Mortal Kombat with Wells Gardner 25K7197 25" monitor
$34 WG K7000 Flyback repair kit
$1.50 1 pk 555 lamps

PC
$80 ish Arcade VGA card
$55 PC Chips mobo
$13.78 TAEC floppy drive
$10 CPU fan
Free donated Athlon XP 2500+
Free canabalized RAM
Free canabalized HD
Free canabalized Power Supply and case
Free canabalized DVDRom drive
Free keyboard and mouse

Interface
$73 J-Pac
$46 Replacement control set for MK
$7 1 Bag of 20GA 10' x 10 colors (100' total) wire
$5 4 .100 Housings with pins 12 pos
$4 2 Midway 12 pos double headers

ROMS
$11.50 Complete MAME .100 ROM set

Tools and Misc.
$3 Charging clips (alligator clips) (for monitor discharger)
$1.50 4ft. of "lamp cord" power cable (for monitor discharger)
$6 random shipping charges
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So lets say somewhere in the ballpark of $700. I'd say $800 is a good estimate of what it would cost to get running from the ground up if you had nothing.