I don't know about Railgun being expensive, because walking tanks and all these technologies that Human are using in the manga are even further out of reach than Railgun, and the test Railgun is actually of low caliber. Railgun with large enough caliber and high enough velocity is very powerful, a simple showing is in Transformers 2 http://www.youtube.com/watch?v=ZJVdHhlLcTw
. Sure, that's also fake but that's how they should be in this manga. Then you have Mass Driver, which is quite close to Railgun in principle but on grander scale, the primary space ship weapons used in Mass Effect series (which is relatively "hard" sci-fi). Those things is equal to or much greater in term of firepower compared to those Super Heavy Cannon the Fog used (I remember reading from ME1 that a shot from the Earth's Battleship equal to the Fatman explosion, but much more concentrated). The reason why sci-fi usually opt to showcase Energy weapon isn't because they are more powerful, but because they are shinier and eye-candy, especially using underwater condition they are even less effective (reflection).