Automate testing of Silverlight application : MS Test Manager vs Selenium
Actually you don't need the Test Studio, hence the Telerik Testing Framework is free. I did used it for Silverlight and with a few tutorials you'll be up to speed in no time.
Most valuable key features are - Rich-API (Testing Framework exposes numerous properties and methods to help you easily build non-brittle, maintainable functional tests) and Set implicit and explicit waits. Also Cross-Browser Compatibility is well supported.
Another very used tool is Sikuli. It automates anything you see on the screen. It uses image recognition to identify and control GUI components. It is useful when there is no easy access to a GUI's internal or source code. So..flash, desktop etc. The JAVA API is very useful.
Would you be willing to consider a different tool? Test Studio can test Silverlight applications.
Disclaimer: I work for Telerik.