I would start here
http://www.teamxecuter.com/ and check out all of the "Official Xecuter Stores" to the left and price from there.
If you want to buy your own hard drive and install it, you could probably save a few bucks, but then you have to crack your case to do it. It might be easier just to get an upgraded HD through the store. I would figure on ~$400.00US
Keep in mind that I haven't done this yet, just my best estimate from the reseach I've done.