[{"Agno":"2013","Egresos":[{"Fecha":"28-01-2013","Monto":150000,"Detalle":"Pago Nokia Lumia a @josellop"},{"Fecha":"29-01-2013","Monto":4000,"Detalle":"Cine, Pelicula fome"}],"Ingresos":[],"Mes":"Enero","Monto":450000,"Usuario":"MarioCares"}]

db.collection.aggregate([{$match:{Usuario:"MarioCares"}},{$project:{suma:{$reduce:{input:"$Egresos",initialValue:0,in:{$add:["$$value","$$this.Monto"]}}}}}])