To Bart's point thoughif you can get three times the reloads out of SC brass aren't you money ahead with the SC using your formula?
It would be if you were certain of getting all your brass back.
In may case I never do.
Conversely, I could choose to pick up my 9MM brass which costs 1/3 to 1/4 th the price. I personally havent tried to get more than 2 loads from the same brass, but others here have commented on being able to get 3+ relods from the same cases.
Once again it depends on your circumstances. Used 9 MM brass will most likely always be cheaper than new 38 super.
However other things like the price of converting an existing gun that is already in 38 ___ , the price of a casepro, the value of your time re-sizing 9MM brass that may have passed thru a Glock, whether you have a large stash of brass of either caliber, will weight decision one way or the other, what powders you can get your hands on (my 38 super top end shoots much softer with VV 3N38 than my 9 MM top end in shooting Sihlouette).
Good luck in your decision and remember wherever you go,, there you are.
Robert