The cost varies if you want a simple ceremony, which still involves paperwork for the church, administrative costs, and the working time of the church staff and the priest, you can give the church a free donation. It is good to book in advance.
We need to move in advance, for any church, there are many Italians and foreign spouses who wish to get married in the Vatican or in the church they prefer.
If you want a professional who takes care of the booking part of the church, the paperwork and the administrative part, and you also want an organist and the flowers and a photographer you have to rely on a planner.
If you have other questions please write me.