There are quite a few so-called "tactical knives" on the market. Many of these knives have handles large enough to extend below the hand, exposing the butt. Any of these are likely strong enough to use as an impact weapon if needed.
I often carry a Spyderco Police with the all steel frame. It has a large handle, and the all steel construction gives it a bit of heft. It would certainly get someone's attention if I had to hit them with it. It will also break auto glass if necessary. It's a bit more than $100, but there are lots of other choices.
Bram Frank Lapu Lapu Corto is designed as a striking knife, there is also the Jim Wagner Reality-based knife. They both need some time to get used to unless you are already familiar with FMA/Silat concepts.
I carry a Spydie Yojimbo, but it'll cost more than $100.
mmm... practically any large framed knife would leave some frame/steel exposed for some strikes, but i like the heft of my CQC 15 for striking. I have a Spyderco Gunting which fits the bill nicely but she has been retired to safe queen status.
Spyderco Police model. The Stainless handle models are thin, strong and would put a serious dent in anything you hit. Latching your index finger around the hump really locks the knife in, resulting in very little back slide on impact.
I was going to mention the Gunting, unfortunately no longer in production but it was basically made exactly for this. Much controversy about the concept (i.e. if you wound up having to stab someone the fact that it had non or less-lethal striking points could come back to haunt you in court--i.e. why didn't you just smack him with the butt instead of stabbing him?
That Wagner reality-based looks like a mean mofo, anyone held/used on? How's the quality?