Here are some hints:
First of all, DON"T USE SUPERGLUE!  Many times people will decide they can
superglue the broken part back on.  That succeeds in gluing the lock
together.  I'd suggest lubricating the keyway liberally with WD40.  Then
break the end off a coping saw blade and see if you can snag the edge of the
key with that.  Can you stick the broken part of the key into the cap and
unlock it?  If so, the roll pin can be driven out and the cap dismantled.
Generally you can them push the broken part out from the rear, unless it is
hanging on one of the wafers internally.  A local locksmith can do this very
easily for you, and generally from the outside of the cap if you can't get
it off.  That will be cheaper than replacing the cap.  I generaaly charge
$7.50-$10 to remove broken keys from the locks.  Local prices may vary, but
it will still be less expensive than a replacement.  Good luck!
Steve Sandlin
Steve's Lock Shoppe
'84 Wolfsburg Westy
'79 Transporter
'00 New Beetle