I feel somewhat qualified to answer this as I have owned both.
The primary price difference is in the finish. Polishing takes time and costs money, as does the fine blueing of the barrel on the BGH. The finish on the X2 is not a park or phospate, it is paint/coating much like the finish on the 870 express. Wood stocks cost more than synthetic to make if you were looking at a wood BGH. With the BGH you get a "speed feed" feature that allows you to load one in the magazine tube with the bolt open and it will automatically chamber that round. It is faster and it works. With the BGH you also get a magazine cutoff, this will allow you to empty the chamber without a round being fed from the magazine. This feature is not worth much to me. Popping in a goose load when one strays too close to you while hunting ducks for example is about the only use. Features cost money. The two guns are also targeted at different hunters, price is partly set by marketing people to satisfy those that are not happy with what they bought unless it was more expensive than the other gun.
The gas system on the guns is the same, but a mirror image. With the X2 the op rod is on the left, whil on the BGH it is on the right. The "features" of the BGH do add complexity to the gun, which COULD decrease reliablility. In hardcore waterfowl circles the BGH doesn't have a very good reputation, while the X2 has a great reputation. This could be from several things but IMHO it is because all the teething problems of the basic design were in the BGH, it came out several years before the X2.
I sold my BGH to buy an X2. My reason? I could not bring myself to paint over the wood and blueing on the gun, it was too nice. I also prefer the higher rib on the X2. Both are dead nuts reliable. The BGH tended to slow down before the X2, it needed to be slightly cleaner to be happy. This could easily reverse with two more guns though. Both are good guns, buy the one that fits you better.
Hope this helps.
