Laravel 5 trying to write query to subtract one column from the other and sort by the result

I'm trying to subtract one column from another and sort by the new AS variable. I suspect my 'minus operation' isn't correct in my query. It fails with the message:

Unknown column 'used_vehicles.selling_price' in 'order clause'

My controller:

public function showUsed()
{

$vehicles = DB::table('used_vehicles')
        ->join('dealerships', 'used_vehicles.dealer_id', '=', 'dealerships.id')
        ->join('makes', 'used_vehicles.make_id', '=', 'makes.id')
        ->join('models', 'used_vehicles.model_id', '=', 'models.id')
        ->join('used_vehicles_categories', 'used_vehicles.category_id', '=', 'used_vehicles_categories.id')
        ->select('used_vehicles.vehicle_price - used_vehicles.discount AS used_vehicles.selling_price')
        ->where('used_vehicles.archived', 0)
        ->orderBy('used_vehicles.selling_price', 'ASC')
        ->paginate(30);

    return view('/pages/used/index', ['vehicles' => $vehicles]);

}

Any idea what is wrong?

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