You still have to pay extra for the extra Strength rating, as well as the base price of the bow itself, including any other modifications you may want to make to it, like making it out of Darkwood. The trait just saves you the cost of Masterworking it, gives you a +1 bonus to attack rolls with it, and gives you a limited proficiency. Still pretty good for a trait.
Is there any limit to the amount of non-magical extra bling you can have tacked onto your free mundane weapon with this trait?
From what I remember reading the other day, all you get is a masterwork weapon for the cost of a normal weapon no? You would still have to pay for anything above and beyond masterwork quality (as in the str mod addition to composite bows). So no the only limit is how much you can afford at first level/when you get the trait. The cost of masterwork is all you get for "free" from the trait.
As above... you want Str bonuses, Darkwood etc then its 'user pays'.
They get the MW free only.