What is the value of a 1928 US 500 dollar bill?

If you have the more common green-seal Federal Reserve Note, as of 12/2010 it's worth $750 to $1,400 if circulated, and over $2,000 if uncirculated.

The much rarer 1928 $500 gold certificate retails for $4,000 to $12,000 in circulated condition and at least $25,000 if uncirculated.

In either case your bill should be examined in person by an expert to get a more specific value.