docval2

secondary> db.orders.aggregate([
    {$match: {
      price: {$type: 2}}},
    {$group: {
      _id: "$price", 
      count: {$sum:1}}}
  ])

{ "_id" : "if you have to ask....", "count" : 250 }
{ "_id" : "free", "count" : 500 }