My first DK was a cocktail. Paid $350 for it (the guy was asking $500) in 2009. Sold it a few years later for the same price.
Considering how the game could be parted out, you might conservatively make $100 for the game circuit boards, $100 for both joysticks (if they're original Nintendo sticks), $100 for the Sanyo CRT monitor, and $50 for the Nintendo arcade power supply. IMO $350 is the sum of its parts, and a reasonable price for the machine. YMMV.