Is there a good "rule of thumb" for translating EXPLAIN cost to (wall-clock) runtime?
I suspect that this is not possible the way you imagine it.  One important reason is that the actual run time is very hardware dependent, and many of the optimization decisions that the database engine does are effectively about balancing the use of the different hardware components (e.g., disk, memory, CPU).

I suggest that you run a bunch of queries relevant to your application, record the cost estimations and the run times, and try to make sense of that data.  You might get a nice linear relationship, or you might learn that the cost numbers are worthless for this purpose.