Former Corbis employee here (2003–2008):
There are a few differences. The images you get from Corbis have been cleaned, color-corrected and/or white-black balanced, etc. The resolution is often higher than the public domain scans. Sometimes Corbis has the original negative, while the public domain relies on a second, third, or higher generation copy. In other words, the Corbis quality is almost always much higher.
I’m not sure that justifies the high price, but from a legal point of view, Corbis’s corrected and balanced photo is their own unique digital version and can be licensed as such.
I often mentioned the public domain version when dealing with independent and non-profit agencies. Some were very appreciative, others didn’t like the inferior quality and would license the Corbis image instead of saving money.
