http://dieselgiant.com/repairignitionlock.htm
The above link is a great place to start.
You may not be as lucky as it's author, but if that is the case you will have already figured out where to start cutting... you'll want to leave the plastic plug connector in the back un harmed...
A new tumbler (including 1 new key) runs about $80.00
The other parts of the steering lock/ignition switch assembly total up to about $150.00 ...but if you can get them out of a junk yard that has a key for the car (otherwise the labor costs will go up) you can probably get them for less than 50 bucks. In my case they diddn't have the key and I was forced buy a new armored cover and cut theirs off.
Best of luck...
Judging by the number of tumblers going bad this summer, maybe there should be a stickie about this... then again I have a feeling it will do limited good overall.