What's the cheapest way to get the training needed to become an IT system admin?

The cheapest way to buy the books and study it yourself, then get certified.

Or better yet, read them at the library and then get certified.

Your only other cheap choice is taking classes at a community college, which tend to charge less than other places of higher learning.

Books plus an old computer plus trial and error.

Depending upon what state you are in, there may be a considerable amount of support you may be able to receive from institutional or government sources. I would check with your local workforce for economic development Council for more information. Absent that, consider interning with a local IT support firm. Small companies like this often gladly accept the help and it’s a great way to build that first line item on your resume. If you are interested in seeing what types of benefits are customarily offered in this line of work check out this link. Bear in mind that starting wages are usually low, but advancement opportunities abound so you can usually move up quickly if you are good.

Try to find a friend that does it and create your own together. Just make sure you are passionate about it.