What is the value of a 1928 US 20 dollar gold certificate?

As of 05/2014 approximate prices are:

Circulated: $60 to $125 depending on the bill's condition

Uncirculated: $300

Serial Numbers

A bill's serial number is a counter and a security feature. In most cases it doesn't affect a bill's value or help to identify it. Some collectors specialize in bills with low serial numbers (e.g. 00000005) or unusual patterns (12344321) so these can bring above-average prices.

DISCLAIMER: The values quoted are market averages as of the date shown, but may be different for an individual bill due to variations in quality and other factors. Also the wholesale (buying) price of a bill will be less than the selling (retail) price. A reputable currency dealer will be able to give a more accurate valuation based on an in-person inspection.