They start to lose interest when you start thinking about "production." Making a whole load of something that works sounds like a great idea to me. The per-rover cost would be almost the same, since everything is custom-built, but a lot of the research is already completed.

I've got a question for those who know the rover's design a lot better than me:

If you dropped one of these on the Moon, would it still work? I doodled up a landing system that scraps the parachutes and heat shield and beefs up the retrorocket braking system. Airbags would still work, if they have enough pressure margin for the difference between Mars atmosphere and vacuum.

Obviously, all the electronics are rated for near-vacuum, but temperature excursions would be the big showstopper IMO. Extra heaters could only go so far without solar power--a lunar night would probably kill an MER, or at least some of its functions.

Worth the trouble to modify the design, or are so many mods required that it is just as easy to design a Moon model from scratch? I frigging can't believe we haven't put another rover on the Moon years ago (Russians had some).