Hello John,
I suggest you set the price determination for by-product to 3. This will force you to have price control S for it.
Since you have price determination 2 it is not taking part in actual costing.
At end of month, review the Material Price Analysis for the month and enter a value in MR22
to get the Cumulative Inventory value to the market price determined for the month.
Then when you run Actual costing run, I expect it will allocate cost to consumption and ending inventory.
Hope this helps.
Siva