I urge caution against choosing an existing generic expression evaluator over a purpose-built math evaluator. The reason for this is expression evaluators are not limited to math. A clever individual could use this to create an instance of any type in the framework and call any method on the type, and that would allow him to do some decidedly unwelcome things. For example: new System.Net.WebClient().DownloadFile("illegalchildpornurl", "C:\openme.gif") will evaluate just fine in most of those, and do just what it sounds like it would (and make you a felon at the same time).
This doesn’t mean don’t look for something that’s already written; it just means be careful. You want one that does math, and only math. Most of what’s already out there isn’t that picky.