As you've discovered, it does vary a lot, so it's hard to give exact figures before we know exactly what we want to add. However, you can see how much it cost for our last set of servers in this post about the new server setup (http://archiveofourown.org/admin_posts/109). The most expensive one was US$5,554, so that gives some kind of guide.

Hope that helps!

Lucy, AD&T staffer