The sword of subtlety does an unusual thing, so it's pricing was pretty much done completely ad-hoc by comparing it to similar items and then settling on a price that seemed about right when compared to other items.
You can change it to any other weapon without much fear, though—in fact, changing ANY of the unique weapons to any other weapon is such a minor change that pricing doesn't really care; when an item costs in excess of 10,000 gp, the change to the weapon's base price (dagger to short sword to flail to whatever) is negligible.
Is there some recommended method you could suggest to figure out the cost of say, increasing the enhancement bonus on the sword from +1 to +2? Or perhaps just considering "subtlety" to be similar to some of the armor enhancements that do not affect enhancement total, and only incur a fixed cost?