The helicopter uses a Qualcomm Snapdragon 801, which doesn't appear to be radiation hardened. I'm guessing their usual BAE RAD750 PowerPC rad hardened CPU was too heavy to put on the helicopter.
You need a specialized fab manufacturing, etching and packaging process for this and it's not like regular fabs are cheap to begin with. Plus you're working from the start with much larger nodes with dedicated cell libraries so everything has to be designed from scratch to fit that node which means you can't reuse consumer off the shelf designs very easily.
Then there's the lack of economies of scale in building such custom parts in small numbers. I imagine if Apple would only order 100 5nm chips per year from TSMC, the unit price would be equally eye watering.
seriously, that's something I haven't thought much about. when designing rovers, do they calculate solely on the weight of what it will be on the destination, or limit it to weight limits of escaping earth's gravity well?
Launch mass is important as a constraint, but the launch environment is the main design driver for mass. The rover must be much stiffer than the rocket to not “couple” it’s response. The first vibration mode (think tuning fork) of a rocket is about 20Hz so the spacecraft inside needs a first vibration mode higher than 40Hz. Something inside the spacecraft similarly needs a first vibration mode higher than 60Hz or 80Hz, although you can make exceptions based on analysis.
But that’s not all, the sustained load on components during launch can easily be 10 to 30 times gravity (30G) and the instantaneous load can be 100G. You can’t just add material for these kinds of loads, it would be an endless feedback loop because adding mass decreases stiffness. Look closely and you’ll see that every deployable or movable part of the rover is locked down by a mechanism until it has landed.
I'm guessing the faster CPU is just not necessary for the core rover, so via KISS, use the proven chips.