Prueba así a ver:
Código:
CAST(SUM(PVP * UNIDADES) AS NUMERIC(9, 2)) AS BASE,
CAST(SUM((PVP * UNIDADES) * IVA / 100) AS NUMERIC(9, 2)) AS IMPIVA,
CAST(CAST(SUM(PVP * UNIDADES) AS NUMERIC(9, 2)) + CAST(SUM((PVP * UNIDADES) * IVA / 100) AS NUMERIC(9, 2)) AS NUMERIC(9, 2)) AS TOTAL