Generally, this is a much simpler document than the motion itself, since we only need to respond to their opposition. Either they are right or wrong, and the Reply points out al the ways that they are wrong. For the judge, it's our last word getting in before he rules on the motion.
However, the minimum charge is for the standard motion. Due to the complexities of what might be involved in the Reply responding to their Opposition, and there are many different and compounding types, more time may be required than the expected hour to provide the effective reply for you.