I bludgeoned this to death with a subquery that returns the correct results as follows:

SELECT M.BoMWOID, m1.BoMTotMaterialEstimate,

CAST(SUM(D.BoMItemUnitLaborHrs * D.BoMLaborComplexity * D.BoMEItemQty) AS money) AS BoMTotLaborHrsEstimate,

CAST(SUM(D.BoMItemUnitLaborHrs * D.BoMLaborComplexity * D.BoMEItemQty * DI.BudgetHourlyRate) AS money) AS BoMTotLaborCostEstimate,

CAST((sum(D.BoMItemUnitLaborHrs * D.BoMLaborComplexity * D.BoMEItemQty * DI.BudgetHourlyRate) + M1.BoMTotMaterialEstimate) AS money)

AS BoMTotCostEstimate

FROM dbo.tblBoM AS M INNER JOIN

dbo.tblBoMDetail AS D ON M.BoMID = D.BoMID INNER JOIN

dbo.tblWorkOrders AS WO ON M.BoMWOID = WO.ID INNER JOIN

dbo.tblDiscipline AS DI ON WO.WODiscipline = DI.ID LEFT JOIN

(select BoMWOID,sum(BoMEstimatedMaterialCost) AS BoMTotMaterialEstimate from tblBoM Group By BoMWOID) as M1 on M.BoMWOID=M1.BoMWOID

Attached is an image of the table structures and relationships:

GROUP BY M.BoMWOID, m1.BoMTotMaterialEstimate

Is this the best approach?