[{"date":"12-2-2015","cost":100},{"date":"13-2-2015","cost":10},{"date":"12-2-2015","cost":40},{"date":"13-2-2015","cost":30},{"date":"13-2-2015","cost":80}]

db.collection.aggregate([{$group:{_id:null,orig:{$push:"$$ROOT"},"total":{$sum:"$cost"},}},{$unwind:"$orig"},{$project:{date:"$orig.date",cost:"$orig.cost",total:"$total"}},{$group:{_id:"$date",cost:{$sum:"$cost"},orig:{$push:"$$ROOT.total"}},},{"$unwind":"$orig"},{$group:{_id:{_id:"$_id",cost:"$cost",total:"$orig"},},},{$project:{date:"$_id._id","cost":"$_id.cost",total:"$_id.total",_id:0}}])