So a +5 Main Gauche would typically have the +5 count only for the OB, right? Would it be possible to purchase a +5 Main Gauche that has the +5 count only for the DB? Would it cost the same as a "normal" Main Gauche? And would it be possible to purchase a Main Gauche that gives +5 to OB and +5 to DB? If so, what would it cost?
Yes, normally such bonuses are for OB only (which also allows the Bonus to be applied to DB through parrying -- which is something
completely different than the shielding bonus that such a weapon has).
Shielding Weapons (like the Main Gauche and Sai) get their DB bonus
from their design and how that design is used defensively (which is why it ONLY applies when not attacking with that weapon).
Now, personally, I don't think that special materials would increase that "design/usage" bonus to DB. Though special materials can most certain increase its OB (i.e. harder materials making it easier to penetrate armor).
Workmanship, is a bit more iffy. It requires a judgment call on the part of the GM as to whether or not a craftsman can improve on the design enough to grant it a bonus (personally, I would require 2 bonus increments for a single +5 to defensive side of the item).
Now, a truly magical bonus (via the Imbue Bonus spell) can give the weapon a magical (not based on material or workmanship) bonus to DB, but that would give an overall modifier to DB whenever the item was held (i.e. drawn, not holstered). Then again, I would also make it more difficult to enchant an offensive item with a defensive bonus as well.
As for costs? Those would be setting dependent, and thus up to the GM.