How much does it cost to replace an ignition switch on a 2001 Cadillac Deville?

Guess-ti-ment .. $100 to $ 150. You can buy new cylinders on e-bay, either assembled with new keys or un-assembled in pieces with no key. If you want a new key for just the ignition, then get an assembled one. If you want to continue to use your existing ignition key, buy the one in pieces. Then take the cylinder in pieces & your best ignition key to a local locksmith, & he will assemble the cylinder around your key. Now either way, you have an assembled cylinder with working key. The directions on removing the old cylinder are usually with the new cylinder, & involve drilling a small hole into the face of the old cylinder to break a snap ring that is holding the cylinder into the steering column. Remove the bad cylinder & snap-in the new one. On the newer cars with the chip in the key - Do the same, because the sensor that reads the key is not part of the cylinder itself, but in a ring around the cylinder.