select CASE
    WHEN estimador.Clave = 'Perfiles' THEN (select gasto.[Importe Banco]*0.5 from SN_INT_PartidasGasto as gasto where gasto.[Clase de Coste]='FSW' and estimador.IdProyecto=gasto.[ID Proyecto] and AmbientePRE='PRE')
    WHEN estimador.Clave = 'Puntos V1' THEN 'Puntos V1'
    WHEN estimador.Clave = 'Puntos V2' THEN 'Puntos V2'
    ELSE 'Sorry, no match.'
    END AS CATEGORIA from tb_Estimador as estimador where estimador.IdProyecto in (select gse.FolioSantec from db_Incurrido.dbo.GSE_Real as gse where gse.FolioSantec=estimador.IdProyecto and estimador.Clave!='Tarifa Promedio')

Msg 512, Level 16, State 1, Line 1 Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= ,
>, >= or when the subquery is used as an expression.