I don't think that Nicodemus would go through all that trouble, especially with sacrificing his daughter, if the grand prize was an item which had a slight improvement over the noose already around his throat.
Honestly, with this guy, I think there's only one or two things that he would give about anything for. My best guess is that the knife (if it IS the same spear which pierced Christ's side) is a conjunction, much in the same way that Halloween is - it's an item which allows the user to slay an immortal. Only instead of having to wait for a particular time, or approaching a being while IN the flow of time, it just gives them the ability to kill.
No real evidence, but it fits thematically both with who Nicodemus is, the small list of impossible things he'd sacrifice his daughter in order to accomplish, and known potential powers given the history of the item in question.