There is 'off the shelf', 'automotive', 'aeronautic', 'unmanned space' and 'manned space' quality gear.
If you think about it - when you're flying a plane, the 'tolerances' for everything just has to be higher - there's much more at risk.
In a complex system, there are a lot of single points of failure that can cause haywire.
Unsurprisingly, the tolerances for 'unmanned' vs. 'manned' space flight are quite different - and for good reason.
Perhaps the economics are not always quite right, it may make sense to make some small probe out of 'cheap gear' - but there is of course added risk, and usually, you 'only get one launch' attempt.
Finally - there's a reason a 'space grade' radio might cost $13K, and that's because they don't make very many :).
I interned on the Space Shuttle robot arm project - we made these aluminum 'handles' for the arm to grab onto. I mean - really, really basic stuff. $200K each. Engineering, overhead, testing, sales yada yada - we only ever made a few dozen.