Yep, that radiation hardened 486 was used quite a bit. I believe it was used for the Space Shuttle as well.
The Mars rovers have moved to a more efficient PowerPC platform, but its still old tech. Radiation hardening CPUs isn't cheap and the processing requirements for what these projects do isn't high (its more sane to just send the data to NASA and have terrestrial computers do all the heavy lifting), not to mention you don't run a bloated desktop OS on these things. A minimalist VxWorks RTOS is typically used.
Price? The RAD750 on the Curiosity Rover starts at about a quarter million dollars.