This has the best cost-to-damage ratio for a weapon in the game!
4 damage/x, where x approaches 0 (cost of Dirk), is

[infinity]. w00t, go cost-to-damage ratio!
Also, you can't go 4/0, because that's against the rules of math. However, you can make it a limit and make it approach 0, so that the answer
approaches infinity. Calculus ftw.