[{"extra":[{description:"laces",type:"exterior",plus:"200"},{description:"sole",type:"interior"},{description:"logo",type:"exterior"},{description:"stud",type:"exterior",plus:"450"}]}]

db.collection.aggregate([{$project:{extra:{$reduce:{input:"$extra",initialValue:{cheap:[],exp:[]},in:{cheap:{"$concatArrays":["$$value.cheap",{$cond:["$$this.plus",[],["$$this"],]}]},exp:{"$concatArrays":["$$value.exp",{$cond:["$$this.plus",["$$this"],[]]}]}}},},},}])