This is the rules section, but I am going to give you my thoughts on it. I would do the 1/2 first, then apply the DR to actual damage the creature would have taken.
I would do this similarly for energy damage where both resistance (such as from resist energy or protection from energy) and vulnerability might be in play. For instance, if protection from energy (fire) were in effect (30 points of absorption remaining) on a creature with fire vulnerability, I would count a 32 point fireball as 32 points of fire damage first, removing the remaining protection and then dealing 2 fire damage (which increases to 3) on the target. I would not increase the 32 damage to 48 damage before removing the 30 fire protection and then deal 18 damage.
As for whether there is an actual rule stating the order of application, someone else may know.
