It's funny because I was actually thinking about this exact scenario the other day. I asked myself, "How much would it cost to build an entire DK from scratch..piece by piece"
I started off by vising Mikesarcade.com and adding up each part in an Excel spreadsheet. I got half way done and quit. Each part by itself was not expensive. But what I quickly realized is how many parts are actually in a DK machine. The cost quickly eclipsed the price I would ever pay for a used machine on Craigslist.
But it's a fun exercise and I encourage you to do it. Go on mikesarcade.com and start adding it up on a spreadsheet. I think you will be legitimately surprised how fast it all adds up...and that is before shipping cost.
Between the coin door, power supply, pcb, joystick and monitor you are looking at $525 and that is a conservative estimate.
In other words, this situation is called a paradox. The parts are worth more than the whole machine.