[{"_id":1,"item":"Item 1","price":"10"},{"_id":2,"item":"Item 2","price":"20"},{"_id":3,"item":"Item 1","price":"5"},{"_id":4,"item":"Item 2","price":"10"},{"_id":5,"item":"Item 1","price":"5"}]

db.collection.aggregate([{"$group":{"_id":"$item","average":{"$avg":{$toDecimal:"$price"}}}},{$project:{_id:1,avg:{$round:["$average",1]}}}])