For those who think they are going to try this watch out.  1) printers are networked and guess which codes go straight to you know who if you try to replicate the image and 2) printers microembed their own codes into ink, both inkjets and laser.  So if you bought your printer on credit, you'll get hosed.

Copy paper doesn't pass the starch pen test.

But, I'm more interested in the dynamics.  If I want to purchase 10 pounds of coffee and I offer the grocer a cocktail napkin with "$100" written on it (assuming the grocer accepted it) no one would have a complaint.  If I do the same thing with a napkin that had previously read "$5" there's still no complaint.

Why is a cocktail napkin potentially worth more than a $5 bill?

Good lord, why?!  The newer $5 bills have about as much quality and feel as something I would print on my printer.   Might as well just print them.