Theoretically you're right, but you'd need some super high density material to prevent it from shattering on impact.Aren't railgun slugs just.....hunks of magnetic material? That's kind of the whole point of a railgun, it just shoots something fast as fuck.
I'd hazard a guess that the ammo for a railgun would be cheap, but the gun itself would be expensive to acquire and need some pricey maintenance (new barrels every X shots or whatever)
More importantly, I think from a balance point of view, CIG wouldn't want inexpensive super-long-range weapons since people would just try to turtle with those and defeat the risk vs. reward thing.