Thanks Michael. I've decided to track the item costs in the fact table, and use the ETL scripts to split costs into material/labour/overhead costs.

There is an item cost on the item header in the ERP, so I won't need to do any cost calculation, regardless of the costing method used (even if it changes). I'm going to write a line to the fact table which is for a zero-quantity transaction but shows the cost breakdown of the item at the end of each day. This will cover the situations where there is a change in item cost but no transactions against the item over a few days, and also ensures I get the standard cost as opposed to the current cost or some other doctored cost (which is used on the odd occasion).