Killebrew is not a tough signature. That helps drive prices more so than the players popularity and HOF status (Kell, Doerr, Snider, Feller and few others come to mind as well for this category, although Killebrew tends to be a tier or two above the easiest of the easy as does Snider for his certified stuff).
As with the unusual GU cards of subjects not often used (Arky Vaughan, Joe Cronin, Rogers Hornsby, etc.; the prices for these unique cards are reflective of the demand for shorter supplied cards.
I remember seeing someone here boasting that they nabbed an Otis Nixon certified autograph and I want to say the price was in the high double digits! This was based only on the fact that it was either his only certified issue at the time or perhaps it was his only certified issue featured on that team. I can't remember now, but I found it hard to believe until I looked up prices of similar cards and saw it was true!
Your pick up is a nice one for sure. Clean design and an ever popular on card signature as well. Congrats.