The biggest thing is that SafeRTOS has gone through the certification while FreeRTOS hasn’t. My understanding is that SafeRTOS IS a slightly different codebase (I think maybe even removing some functionality) with the changes being what was needed to pass the safety standards.
The cost of SafeRTOS is in part to pay for the work that went into that certification.
You are of course free to take FreeRTOS, do the necessary processes to certify it to EN 62304 (possibly needing to make some small modifications) yourself. I suspect if you look at the costs, SafeRTOS is likely cheaper.