I’m afraid there is no real answer to your question — not one that I could find anyway. It looks like no one has conducted a nation-wide survey on the average cost of plumbing training in recent years.
What I did find, however, was ballpark estimations of different costs involved in the process. There is a lot of variation in pricing depending on a variety of factors, but the very least you’ll need to pay to become a plumber is $1900. That money will cover the following expenses:
$800 for an online training program;
$1500 for five years of apprenticeship, including the cost of books and other materials;
$100 to get your state-issued license.
Your training may cost more or less depending on the requirements for becoming a plumber in your state. If you’d like to learn more about this profession, you should check out HowtoBecomeaPlumber.org.