1990 Cadillac Deville key

Does the cylinder rotate, car start, and function normal? Just the key won't come out? The cylinder is serviced by replacement only, if it don't rotate, that will complicate matters.

If it doesn't rotate back to the off/lock position, the key will not come out. You could have something going on inside the steering column, like a broken rack gear or something in the mechanism. Is the column abnormally loose at the tilt pivot, can you get movement by shaking the wheel up/down-side to side?

I don't know what to expect for a cost. It will need some teardown just to diagnose it, and you won't know parts needed until the fault is found. I usually bid an ignition cylinder for approximately 2.5 hours, and that may put you in the area close to identifying the problem. Parts availability may be a concern on a 19 year old vehicle. If I had to guess, it could be in the 500 dollar range, but that is a guess. If you get an idea just to put a complete used column in it, which is a cut and dried possibility, be aware that the theft code may not match, and it won't start.