Well, originality of the cabinet is one thing, but collectabiliy is another... If it were an original cab for another game that nobody cares about, then nobody on the board would make any noise about it... But Robotron is a highly desireable game, and if the cab's in any kind of half decent shape, I'd think hard before shoving a mame box in it.
If you have a free cabinet though, I think it's perfectly reasonable to be able to convert a cabinet for $150US. I picked up a free conversion cab, and my parts budget is certainly less than that. However, if you plan on doing anything unique with the cab (CP overlays, vinyl sideart, marquee), you'll be going way over that. It all depends on what you want to end up with.
Anyways, good luck to you.