New ignition switch with the old key

You can spare the expenses by modifying the new switch to match your original key.

The switch itself is not too complicated to disassemble. First, remove the steering column lower shroud, detach the switch wiring, turn the key to align with the arrow mark between the 'A' and 'S' positions, press the pin on the opposite side and withdraw the unit. Disassemble the lock very carefully and slowly, noting the way the internal parts are put together. There are lots of small parts inside and it would be complicated (although not impossible) to figure out how to reassemble later. Apart from that, it is a simple mechanism.

As in all similar locks (not only in cars), there are pairs of little cylinders (called tumblers) in the lock mechanism arranged in a way to only allow to turn the lock using the correctly shaped key. The pairs of tumblers are pushed against the key by small springs. If you insert the right key, all tumblers will sit in their correct position, allowing the central part [blue in the picture] to rotate while the outer part [yellow] remains stationary. However, if the wrong key is used, some of the tumblers will protrude into the other half, keeping the rotating part from turning.

Use the tumblers from your original lock, placing each into the corresponding location in the new lock: it will then match your original key. Clean the various parts and grease the moving ones as you re-assemble the lock.

If the fault lies in the electric switch of the ignition lock/switch assembly, this is even easier to repair: just separate the switch from your old lock and swap it with the one from the new lock assembly.