Dan G. Switzer, II wrote a plugin for easy calculation. It was recently brought up on the jQuery list (Google Groups | Nabble). Worth a look!

http://www.pengoworks.com/workshop/jquery/calculation.plugin.htm

Update: Dan has posted a blog entry that covers use cases and some of it’s innner workings in more detail, again worth a look! http://blog.pengoworks.com/index.cfm/2008/1/23/jQuery-Calculation-Plugin-Making-calculating-easy