As was mentioned, the intricate machining of high-grade aluminum is the main cost difference. The lockwork of the double action/decocker is a lot more intricate than the glock trigger design, too. I've also heard it said that traditional cut rifling like Sig barrels have is more expensive to cut than Glock's polygonal rifling. And it seems like there are more Sigs on dealer shelves wearing night sights than Glocks.
Next thing you know you're 250.00-300.00 more than Glock. But if you drop 150.00 on an aftermarket Glock barrel so you can shoot lead bullets, and throw on a set of Trijicons for 100-ish, the gap starts to narrow quickly.
It's so easy to tinker with and "upgrade" Glocks that it can be cheaper to buy a Sig!
Eagerly anticipating the next 9 vs 40 vs 45 thread.