I asked this question on Jeeves several years ago and never got an answer. Reason for asking a friend had asked if he could have a pound of twenties for christmas, I’ve never been able to figure it out. Its gotta be a math thing….
5 Answers

Each bill weighs one gram (I actually weighed a 1, 10, and 20), no matter what demonination. There are 454 grams in a pound. So it would take 454, 20 dollar bills, or $9,080. Also the US Bureau of Engraving and Printing agrees with this calculation, check out the link.

20 Dollar Bills

To the best of my calculating it will take 448 $20 bills to weight 1 lb. according to the fact that on my Gemstone scales 7 $20 dollar bills weight in at 0.250 of an ounce that makes 1 oz. contain 28 $20s and there are 16 ozs in a lb. so 16X28=448 hense 1 lb. of $20 dollar bills should equal 448 $20 bills
Source(s): weighted on a TOYO 250 GEMSTONE scale 
One bill weighs .0348 ounces, hence 16 ounces divided by .0348 equals 459.770114 times $20 equals $9195.00. Can I be your new friend. I only want half a pound.
Tom (your new friend)
Source(s): http://www.frbservices.org/Cash/CashQA.html#7 
C, because I would feel bad if I took it, but at the same time, if you said you found a $20 bill and told people someone might end up lying and saying it was theirs,LOL. So, I would just leave it alone.