Though simplistic in nature, what you could do is create an IBA on WTPart for cost, and give it a numeric value (or number with unit, at dollars). As you build BOMs of WTParts, you can configure views/reports/etc so that you can see the cost of every part within a particular part's BOM, then export that to CSV for opening in Excel and performing a summation on the objects' costs together.
Something more elegant than that: you could have two cost IBAs, where one is a manual input and the other is generated by summation of objects' values from the manual inputs within it's BOM, and that being done by WC every time an object is checked in. This would definitely be a customization though and could get complex with multi-level BOM structures.
