Laravel 5.4 Query Builder SUM() from two different tables

I'm having problems getting the SUM of a certain field from two tables.

So to start things off, here's my query.

        //Getting of Cost of Goods Sold (Menus)
        $totalMenuCost = DB::raw('(SUM(orders.qty * orders.cost)) as cost');
        $yearMenuSold = DB::raw('YEAR(orders.created_at) as year');

        $menuscost =  DB::table('orders')
            ->where('status', 'served')
            ->select($totalMenuCost, $yearMenuSold);

        //Getting of Cost of Goods Sold (Items)
        $totalItemCost = DB::raw('(SUM(purchases.qty * purchases.cost)) as cost');
        $yearItemSold = DB::raw('YEAR(purchases.created_at) as year');

        $itemcost =  DB::table('purchases')
            ->where('status', 'served')
            ->union($menuscost)
            ->select($totalItemCost, $yearItemSold)
            ->get();

And when I try to do return $itemcost. It returns two rows:

[
  {
    cost: "792.00",
    year: 2017
  },

  {
    cost: "1700.00",
    year: 2017
  }
]

I'm trying to make it return a single row but having it added, like this:

[
  {
    cost: "2492.00", // that's 1,700 + 792
    year: 2017
  }
]