IMHO,
Spend the money on ammo. Practice with your gun. In a home defense scenario, you will only be shooting 15 to 25 ft. A red is not at all necessary, if you are confident with the gun. Confidence will come with rounds through your gun.
Just keep in mind, a red dot shows a single point of impact. A shotgun is not really a single point of impact weapon, unless you are shooting slugs.
