What kind of pricing is reasonable for a wondrous item that grants DR/-? Price it like AC (DR^2 x 2000gp)?
A Mantle of Faith grants DR 5/evil for 76,000gp. An item granting DR/- is better then that, so should cost more.
DR/- should be the most expensive DR, so you would need to find out the costs of the lesser DR first. Alignment-based DR could be priced at (DR^2 x 3000gp). That would make an item with DR 5/evil cost 75,000gp, really close to the 76,000gp for a mantle of faith. Figure out a price for the rest from that.
in 3.5 you could get Exoskeleton Armor was a +1 breastplate that provided a DR 5/bludgeoning for 21,350. Bludgeoning is nothing to spit at, but is not adamantite, and no where as good as DR that blocks everything, but it isnt a bad starting point for prices