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

There was only one series of $500 bills printed with the 1928 date so a "C" series letter (indicating a fourth series) can't exist. You're almost certainly looking at the Federal Reserve letter which indicates your bill was distributed through the Philadelphia district.

As of 10/2015 a 1928 $500 Federal Reserve Note has a retail price of about $700 in average condition. A crisp one without any fold marks could sell for around $1200.