What is the value of a 1923 D US 2 dollar bill with a red seal?

There were no series letters on 1923 $1 United States Notes; you may be looking at a plate position indicator or some other "housekeeping" mark.

In any case this is a moderately valuable bill compared to its blue-seal silver certificate cousin. As of 06/2014 approximate prices are:

Circulated: $90 to $150 depending on the bill's condition

Uncirculated: $400

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.