I got the same offer. It doesn't look like we're being given a fellowship, just a TAship. I think UW is really poor (they've been dealing with a lot of political turmoil/budget cuts recently), and the math department has a lot of mouths to feed. They might offer people more money after the visit day, I don't know. For what it's worth, I'm living in Madison now, and it's a wonderful place, probably nicer than Ann Arbor (although I'm biased), and inexpensive (if you look carefully for housing).