So with Oblivion, I bought the collector's edition of the original game, I later bought the Knights of the Nine DLC compilation pack, and also The Shivering Isles expansion. In addition to that I also purchased the official soundtrack off of the Elder Scrolls site.
So in total, my estimate is I spent about $150 on Oblivion and it was worth every cent!
With this in mind, I'll probably do the same thing with Skyrim. What can I say, I'm an Elder Scrolls junkie!
So how much do you think you will spend?