A couple of years ago, I was sort of peripherally involved in a DO-178B Level C project that used MicroC/OS-II. I don't know for sure what the costs were but I believe they were in the thousands rather than the tens of thousands (will confirm that on Monday).
The uC/OS-II folks make a big deal out of being certified by FAA under DO-178B.
Now, I know that FAA != FDA but in general, FAA tends to be more strict than FDA, so I wouldn't think that an FAA-"certified" OS would have any trouble getting past FDA.
Do any of you have any thoughts on uC/OS-II vs SafeRTOS?