I'm looking to update a transaction (Quote) line's Unit Cost field via workflow (sublist action group). I'm already able to update the Unit Price (Rate) and Quantity at the line level, but I don't see an option on Set Field Value for Unit Cost. Does anyone know if there's a way to write to this field via workflow? Obviously trying to avoid a script here, though I know I could accomplish it that way.

did you resolve this? My initial thought is that Unit Cost is coming for the Item record.

I did not resolve it unfortunately - it comes from the Item Record, sure - but it's an editable column field just like Unit Price / Rate, no?