Is there a penalty for failing to turn in the final paper?
Well, that paper is worth 20 points, or 20% of your grade, so yes, yes there is.
Or did you mean would I impose a further penalty? I suppose that might make sense, but no, I will not take off 25 points for not turning in a 20-point paper. What you see is what you get.
Actually, I might be willing to give you 3 free points for not making me grade the paper, but I guess I won't say that.
I am convinced that a lot of students think (despite all evidence to the contrary) that they will get graded just on what they turn in. Get C's on the first three pieces of work (worth, in my calculation, 30% of the semester's grade)? OK, that's a C sewn up, so stop turning in work. And then howl in agony when it turns out that you're failing the course for lack of the other 70%.
Grumble grumble grumble.