And that's not splitting hairs how, exactly? The bottom line of this argument is not one of rules, but of opinions.
Some think limbs are not weapons, others do. Some think there should be a difference in cost depending on the complexity of the weapon, others don't.
The facts are, as Xenon pointed out, that the rules only specify that the weapon focus has to be a melee weapon. There is no mention that making a complex weapon into a weapon focus is any different in terms of cost and availability compared to a simple one. This is entirely a GM call.
If it was my table, I'd let a character begin the game with a cyberlimb weapon focus at the standard cost. To get one made after the game began it would have to be enchanted while not implanted, potentially requiring surgery. Would it be impossible? No. Would it be difficult? Probably not. Why? Because a cyberlimb weapon focus is no different to my mind compared to a cyberweapon weapon focus; in fact, the spur is going to be more effective, so I don't personally see any reason for restricting this at all.