According to the National Center for Education Statistics,

Among teachers who spent any of their own money on classroom supplies without reimbursement, the average (mean) amount spent was $479, and the median amount spent was $297. About 44 percent spent $250 or less, while 36 percent spent from $251 to $500.

As the spouse of a public school teacher, I know that teachers spend their own money on supplies.  But I would like to know more about the details of these purchases.  For example, my wife would spend money on decor, i.e., posters, borders, and other decorative elements, which is much different than spending money to supply students with paper and pencils.  Moreover, she would purchase lessons and instructional materials that saved both time and labor.

These three types of expenditures – decor, supplies, and instructional materials – provide some nuance to the issue.  Decor is completely optional.  There is a reasonable expectation that schools and parents supply basic supplies, depending on the item.  Likewise, there is a reasonable expectation that teachers use their own money for instructional materials.