Laravel Eloquent：计算总价的最佳方法

Im building a simple buy and sell application with Laravel 5.1. Each Buy Model has many BuyDetail which stores bought item quantity and buy_price. I have implement the relationship between table on the Model.

class Buy extends Model
{
  #Eloquent relationships

  public function supplier()
  {
    return $this->belongsTo('App\Supplier');
  }

  public function buyDetails()
  {
    return $this->hasMany('App\BuyDetail');
  }
}

我想计算每次购买的总价格.使用Eloquent 的ORM计算总价的最佳方法是什么？

for now i just implement it like this:

@foreach($buys as $key => $value)
    <?php
        $total = 0;
    ?>
    @foreach($value->buyDetails as $k => $bD)
        <?php
            $total += ($bD['buy_price']*$bD['qty']);
    ?>
    @endforeach

   <tr>
    <td>{{$value->ref_number}}</td>
    <td>{{$value->suplier->name}}</td>
    <td>{{$value->created_at}}</td>
    <td>{{$value->buyDetails->count()}}</td>
    <td>{{$total}}</td>
    <td>
        <a href="" class="btn btn-default btn-sm" title="show">Detail</a>
        <a href="" class="btn btn-primary btn-sm" title="edit">Edit</a>
        <a href="" class="btn btn-danger btn-sm" title="delete">Delete</a>
    </td>
  </tr>
@endforeach