It's more that the cost of "over"-engineering some pieces is dwarfed by the cost of getting an object to space/Mars/Jupiter/whatever.
As an example, curiosity cost about $2.5B [0].
It was flown on an Atlas V 541 which costa about $150M to launch.
So it looks like development and operation of the rover is FAR more expensive.
[0] https://www.planetary.org/space-policy/cost-of-msl-curiosity....