Is it expensive to fix oil in spark plug cylinder

What do you mean by oil in the cylinder? Did you take out a spark plug and fish a mini-cam in there and see a puddle of oil or do you mean you have white smoke coming out the tail pipe and you were told you're burning oil? If a mechanic used the words " your problem is you have oil in your spark plug cylinder" - go to another mechanic that will explain what's happening and get a couple of quotes from different shops.