A USA nickel weighs 5.00g according to the US mint (if you don't believe the US Government see this pic with $10 of nickels). A EU Dime weighs 4.1g. Just start collecting them up and balance them on a key. It's easier to glue them together into a 50g stack but not necessary if you don't have glue around.
IBM Model M - 14 USA Nickels or 70g.
I'll take the results and include them in the database of keyboard switch stiffness, I call this the RipOmeter. This guide also includes other hints if you want to be more accurate but it's more fun if you aren't tainted by previous results.
Editors Note: Be sure to center the stack or Webwit might not accept the result.