The hubcaps I got are just tape and spray paint. Quicker payback (: Good for 3.5% from my own A/B testing.
I've also got a skid plate I put up, probably 50$ in materials and sweat equity.
I don't know how much it's hurting/helping, but I drive on my family's farm a lot and have ran over a lot of stuff that's hidden in the grass. So the piece of mind helps. (: