is replacing a coil a big and expensive job

In the land of auto repair, that repair is not an exhorbitant one. Does the vehicle have a particular symptom (hesitation, check engine lamp etc)? Is it not performing as usual? Typically before we replace any parts, we perform testing to verify where the symptom is eminating from. Once we complete the testing, we inform you as to the cause and to what the cost will be at that point. It is better to test than guess.

If you mean a spark plug ignition coil or a coil pack.....no, neither is a big nor expensive job.