About a year ago, I saw an extremely crappy machine made from a Namco 5-in-1. The cab itself was falling apart, and it had a marquee that had been printed on plain paper. It sold for around $900.
I'm going to an auction today, I'll have to take note of what they sell for.
But, I'm with Peale. Just buy an empty or nearly empty cab. Easiest conversion in the world.