Averaging mid 30s with frequent drops to 10-20 on 720p medium settings on my GTX260. I know it's an older card, but considering Dishonored ran silky smooth, I'm a little disappointed.
You have no right to be disappointed.
If anything, it's a miracle you can actually play it. Do you know what the minimum requirements are? It's pretty much right there at the bottom and SUBSTANTIALLY below the recommended.