Well, the 2x1gb RAM is more expensive cause it leaves you room for expansion. Consider a 4-RAM slot mobo. If u fill all slots with 512MB RAM sticks, it will be 2GB but if u want to upgrade to 4GB, for instance, in the future u will need to get rid of all four 512 sticks. But with 2 sticks of 1GB u will just need to get two more 1 GB sticks.
Also, for many mobos they only have 2 RAM slot, so obviously they will have to buy 2 sticks of 1GB Ram rather than 2x512 RAM.