The Piranha Blade does just the same as the Skaven Weeping Blade and it is one of the most popular weapons for skaven players. The difference is the WB only costs 35 points, however, you must take in consideration that it will be wielded a puny strength 4 and 3-4 attacks skaven with lower toughness and crappy armor, the same weapon performs way better on our scarvets or oldbloods and that is the reason of the higher price. In my opinion it definitely worth the points. If the game was completely fair, same items would have different prices depending on the wielder, but that may be too difficult to balance.