You got good advice above, check the online sources who sell once fire brass and you will easily find your answer.
If you can get 100 .357 Magnum cases for $20 that's $40 for 200. if you can find FMJ .357 Magnum ammo for $40/50 rounds that would be a 4:1 ratio. Of course if the factory ammo costs more the ratio will change. I haven't bought ammo for a very long time so I'm not sure of the prices. Of course SD ammo will cost a lot more than FMJ ammo. Probably in the 7:1 or 8:1 ratio.
BUT, are you sure you want to let go of factory ammo right now. It's hard to find and after you shoot a box you still have 50 once fired cases.