aggregation nested Array element itself in mongodb

You can do this using the aggregation framework. You need to denormalize the "product" array using $unwind operation. From there you need two $group stage. In the first group stage, you group your documents by _id which in your case must be a compound field, and use the $sum accumulator operator to return the sum of price. In the last $group stage you use the $push accumulator operator to return an array of "product".

db.customers.aggregate([ 
    // Denormalize the product array
    { "$unwind": "$product" }, 
    // Group your documents by `_id`
    { "$group": { 
        "_id": { "customer": "$customer", "type": "$product.type" }, 
        "price": { "$sum": "$product.price" } 
    }}, 
    // reconstruct the "product" array.
    { "$group": { 
        "_id": "$_id.customer", 
        "product": { "$push": { "type": "$_id.type", "price": "$price" } } 
    }}
])

Which returns:

{
        "_id" : "cust1",
        "product" : [
                {
                        "type" : "toy",
                        "price" : 700
                },
                {
                        "type" : "cloth",
                        "price" : 110
                }
        ]
}
{
        "_id" : "cust2",
        "product" : [
                {
                        "type" : "toy",
                        "price" : 5
                },
                {
                        "type" : "cloth",
                        "price" : 40
                }
        ]
}