[skywhopper]

The question is... how much does the railgun itself cost per shot? $25,000 per round is cheap enough, but what's the impact of each firing on the gun?

Missles and shells are expensive mostly because they have to propel themselves, but they benefit from the fact that that propulsion system only has to work one time. A useful railgun will need to be able to fire thousands of times without major maintenance before you'll start seeing any cost benefit to cheap ammunition.