A cheaper option might be changing the cassette in the back to something with a wider range, maybe a 11-28 or something.

Also, why change the big ring if you're keeping the small ring the same? Just drop down to the 34 for the steeper hills, keep the 50 for descending.

Edit: but if you really want to limit your gearing:
http://www.jensonusa.com/!zPZm7t7Rxe...1BsRoCM2Hw_wcB