Cannot turn key in switch

I have a 2001 Nissan Sentra, five-speed, with 111K miles on it. I have always kept the routine mainrenance up-to-date, and the only known problem with the vehicle (found in the 100K checkup at the dealer) is a faulty catalytic convertor (it periodically triggers the check engine light).

About six months ago when I tried to start the car, the key would not turn in the ignition. After much jiggling of the steering wheel, taking the key in and out, etc, I got the car started. I purchased some graphite powder and squirted it in the key hole, and didn't have any futher trouble. Until this weekend.

Twice this past weekend I was able to wiggle things around, shake the steering wheel, mutter mild profanities and eventually was able to turn the key in the ignition (on one occasion this was with the steering wheel "locked" and on the other occasion it was "free.") Now, though, I can't get the key to budge.

I have been told that this could be everything from a $30 electrical switch to a $130 dollar assembly or the "black box."

What is the most likely cuplrit, and how do I find a solution without getting ripped off? Is this a repair I can make myself?