Someone here shared this so I'll share it again.
http://www.trapshooters.com/rlcalcadv.htm
On the small end, .410 2.5 inch shells with only 1/2 ounce of shot can be loaded for less than a third of the cost of new off the shelf ammo.
Never could understand how a box of 3" .410 shells can cost close to $20. Guess they don't like to make runs of them.