[{"_id":ObjectId("5d9db4462034bf17454d7d33"),"name":"Product1","cost_oneoff":"1","cost_monthly":1,"margin_oneoff":"1","margin_monthly":"1","price_oneoff":"1","price_monthly":"1"},{"_id":ObjectId("5d9dc2f2d8e17309b46f9b03"),"name":"Product2","cost_oneoff":"0","cost_monthly":1,"margin_oneoff":"0,5","margin_monthly":"0,5","price_oneoff":"0","price_monthly":"2"}]

db.collection.aggregate([{$match:{$or:[{"_id":ObjectId("5d9db4462034bf17454d7d33")},{"_id":ObjectId("5d9dc2f2d8e17309b46f9b03")}]}},{$group:{_id:null,total:{$sum:"$cost_monthly"}}}])