A ground development board would be on the order of $50,000, give or take. You'll also need a chassis and power supply of course. “Also - how do you figure out

A ground development board would be on the order of $50,000, give or take. You'll also need a chassis and power supply of course. “Also - how do you figure out if a processor is going to last for ages?” Really big semiconductor junctions help. Low heat production also helps. And then there's this thing called “heritage”. If it worked on the last fifteen missions, odds are decent it'll work on the sixteenth. Many spacecraft have fully redundant processors boards. Whether that helps you depends on whether you think chip failures are statistically uncorrelated. Which they are, sometimes. But not if there is a design flaw or manufacturing defect.