Is there a way to setup a Decay option so where each time an item is looted in a raid it's value will decay 5% and if sharded it will decay 10%.

I'm afraid not automatically. Are you looking to retroactively change the value as well, or just change the value of the item for future purchases.

I can say with certainty that there's no functionality for that currently. You would have to modify the value of the item manually.

Yea, I was wanting to have the item value continue to drop as it was looted over time until a fixed value was achieved. I will just manually update these after each raid.