Most of the halves I've weighed have been noticably light due to wear. That wear and tear is expected when buying and selling junk silver and the prices often reflect it. That's one of the many reasons why junk silver can sell for less than melt value at times. Silver Eagles are usually bought and sold in BU condition and the mint adds a bit of extra (often up to 1%) to make sure that they aren't underweight to begin with. So finding SE's that are underweight is really rare unless someone has been terrorizing it.
I've bought halves that were worn completely flat before. One lady who had a shop sold them to me for half price. I think if I remember correctly they were about 20% underweight. But to me, silver is silver so they were a great deal. No one else wanted them though because they were "worn coins".
