Ok I think I figured out what I need...
@Inst_ID int,
@status char(1)
SELECT
Inventory.Car,
Inventory.StockNumber,
SUM(InventoryService.FinalCost) As TotalCost
FROM Inventory
LEFT JOIN InventoryService ON Inventory.Inventory_ID = InventoryService.Inventory_ID
WHERE
Inventory.Inst_ID = @Inst_ID AND
Inventory.Status LIKE @status
GROUP BY Inventory.Car, Inventory.StockNumber
HAVING SUM(InventoryService.Finalcost) > 0.00