Able ammo, Cheaperthan dirt, Midway, etc, all have good prices at times.
Dick's, the aptly named chain, had and maybe has the KO Brenneke 5 packs for about $2. They also have sales on other slugs, of course.
For indoor use where the blast is contained, I'd go with a RR load, and worry about the cost after.
In tight spaces, one doesn't hear a shotgun go off as much as feel it.Wear ear plugs AND muffs...