I made something similar out of wood and a marble when I was a kid so I know the theory of this works - so hopefully my 3D modelling does too!
Within the 3D model there is a 12mm 'ball'. You could use a marble of a similar size if you wanted to...
Date Made: September 2013
Approx Cost: (Cost of printing?)
Approx Time: 3 hours modelling (plus printing time)
Step 1: How It Works
Once printed, the ball (or marble alternative) needs to be inserted inside the 'runner'... This should just pop in (as long as the 3d printing material isn't too brittle!?!).
It is the pressure from this ball that will 'grip'.
The angle of the top runner means that 'grip' is adjustable - depending on the thickness of the paper inserted.
So to get the clip to grip, you insert the paper between the bottom (flat) runner and the ball. Then let the ball drop down to the narrowest gap (between top and bottom runner). Then slightly pull the paper back out - this will tighten and jam the ball between the top and bottom runner, holding the paper in place...
Hopefully that makes sense!?! I have done some sketches to try and help the explanation...
Step 2: The Model
This was done using tinkercad and can be found on their website by searching for "Money / Paper Clip" or here: