Expect a $500+ repair bill if the cylinder is frozen. BT, DT, have the scars to prove it. If you could find a shop with one of these nifty widgets (click link below, 7MB video) it would be lots cheaper... but they're probably only common across the pond in Europe:
http://www.w124performance.com/movies/Mercedes/124_lock_cutter.wmv
(skip the first 45 seconds)
