Think I've finally got it.
Query 1:
SELECT tblProjectDetails.projectid, tblVolumes.volume, tblVolumes.product, tblProjectDetails.cutindate, tblVolumes.monthdate, IIf(IsNull([reduction]),([currentprice]-[newprice])*[tblVolumes].[volume],[reduction]*[tblVolumes].[volume]) AS Savings
FROM...