Assuming that fielda and fieldb correspond to DG572B.[DL-Acq Unit Cost]and DG572B.[DL-Welfare Unit Cost] respectively, the WHERE clause you showed earlier should work, so perhaps I am not understanding what you want to achieve
Code:
SELECT DG572B.[DG-Drug Code], DG572B.[DG-Drug Name], DG572B.[DL-Acq Unit Cost], DG572B.[DL-Welfare Unit Cost], [DG572B].[DL-Welfare Unit Cost]-[DG572B].[DL-Acq Unit Cost] AS Expr1
FROM DG572B
WHERE DG572B.[DL-Acq Unit Cost] < DG572B.[DL-Welfare Unit Cost]
ORDER BY [DG572B]![DL-Welfare Unit Cost]-[DG572B]![DL-Acq Unit Cost];