[{_id:ObjectId("653c4c017800342a3bedb82e"),type:"buy",item:555,amount:1,priceEach:100000},{_id:ObjectId("653c4c017800342a3bedb830"),type:"buy",item:384,amount:2,priceEach:70000},{_id:ObjectId("653c4c017800342a3bedb831"),type:"buy",item:384,amount:1,priceEach:50000},{_id:ObjectId("653c4c017800342a3bedb832"),type:"sell",item:384,amount:-1,priceEach:50000}]

db.collection.aggregate([{$group:{_id:"$item",totalBuyPrice:{$sum:{$cond:[{$eq:["$type","buy"]},{$multiply:["$priceEach","$amount"]},0]}},totalBuyAmount:{$sum:{$cond:[{$eq:["$type","buy"]},"$amount",0]}},totalAmount:{$sum:"$amount"}}},{$project:{_id:1,avg:{$divide:["$totalBuyPrice","$totalBuyAmount"]},amount:"$totalAmount"}}])