Approximately how much should I expect to pay a mechanic (labor plus
parts) to change a spark plug in a 2000 Kia Sephia automobile?

probably the $2.50 you have paid to get an answer to your question.

unfortunately the cost is not in doing the job you may be assuming is
needed, but in the experience, training, and additional work  to
determine the source of the problem, and possibly prevent future
damage.  Also, each plug will be a bit different, as to position and
accessability, or there could be rust and other complications.

It would be rare to only need 1 plug replaced with no other problens

About $20.00 you show him which one you want him to change.  Perhaps
$35 to $60 if he has to figure out which one needs to be replaced.