Database
[{"_id":ObjectId("576919b66bab3bfcb9ff0911"),"Cost":"$7,100.00"},{"_id":ObjectId("576919b66bab3bfcb9ff0912"),"Cost":"$14,500.43"},{"_id":ObjectId("576919b66bab3bfcb9ff0913"),"Cost":"$12,619.00"},{"_id":ObjectId("576919b66bab3bfcb9ff0914"),"Cost":"$9,250.00"}]
Query
db.collection.update({"Cost":{"$exists":true}},[{$set:{Cost:{$toDouble:{$reduce:{input:{$split:[{$substr:["$Cost",1,{$strLenCP:"$Cost"}]},","]},initialValue:"",in:{$concat:["$$value","$$this"]}}}}}}],{multi:true})