Those numbers are a great scale of how good a thing is. It's roughly 1-10 scale (Mrylo Dagger stats work out to 1.94 for reference).
So Locustlure is about a 3.3 out of 10 weapon. Seb Dirk is 1.5 at that level. So it's a 5 out of 10 and can backstab better. This is actually is a significant difference. It's worth buying with port money.
To summarize my thoughts on the Seb Dirk: