How can I fix and how much does it cost?

My lock on my golf mark 4 has been cut from the outside and the cental locking still works but is missing a piece where it has been cut.

Any ideas how much this costs and why the alarm did not go off? If i put a key in there it opens the CL and the windows. When I lock it the doors are locked and vica versa when I open it using my fob.

Thanks