operazioni aritmetiche nel generatore di query laravel

Devi usare un'espressione grezza in questo modo:

$products =  DB::table('products')
        ->join('hasDiscount', 'products.idProd', '=', 'hasDiscount.idProd')
        ->join('discounts', 'discounts.idDiscount', '=', 'hasDiscount.idDiscount')
        ->select(DB::raw('products.*,(products.price * discounts.tauxDiscount/100) as newPrice'))
        ->get();

https://laravel.com/docs/5.4/queries#raw-expressions