Start with the three evidence sets
Sales provides the movement and revenue signal. Inventory is a point-in-time snapshot. Cost adds the unit-cost evidence needed for profitability and inventory valuation.
Keep SKU identifiers consistent
The same SKU should identify the same product across exports. Product names help people read results, but the SKU is the practical join key.
Treat missing cost as missing
Missing or partial cost is surfaced as coverage. It is not silently treated as zero, because that would make profit and inventory value misleading.