That item description is
bad: as written, applying enhancements to Bracers of Striking has
no effect whatsoever, and I hope whoever wrote it feels small in the pants. I'm not at all sure what they intended by saying that the bracers count as a double weapon, other than to maybe just confuse matters when you declare you want to kick someone in the nuts or headbutt a demon to death.
I'd advise you to throw the description away and try something more like this:
***
Bracers of Striking: These bracers confer the Improved Unarmed Strike feat upon the wearer. Furthermore, the bracers can be enhanced as though they were a magical blunt weapon, albeit at double the usual cost of enchanting a weapon. The benefits of any such enhancements then apply to the unarmed strikes of the wearer.
***
It'd probably be a good idea to put a line in about whether or not this applies to natural weapons too (whichever you prefer); and to be explicit about the fact that the enhancement bonuses to attack and damage won't stack with Magic Fang and similar spells.
I guess I understand wanting to double the cost: after all, you're giving a character - with the humanoid standard-issue number of limbs, that is - five magical weapons for the price of one. Two-Weapon Fighting applies to unarmed strikes, so someone going this route is getting a bargain if they get the enhancements at the usual price.
Personally, I'd say that somewhere between the two is fair: the armed two-weapon fighter might have to spend more money but they can be more versatile with their choice of weapons (not to mention being able to use two different weapon crystals) than the unarmed fighter with these bracers. On the other hand, don't forget the Snap Kick feat
