It depends on what you’re selling, but roughly $500 per month is seen as solid on the r/PPC forum (that is, no major objections when brought up), with $100 a month as not outrageous (especially as a starting point). My assumption is that the forum is credible and not astroturfed (which seems fairly reasonable from intuition).

I wish there were a scientific study to lean on as a more credible source. I personally don’t have direct experience for budgets smaller than $1,000 a month, back when I did work with some non-profits.

Roughly $100 a month sounds plausibly viable at the lowest uncomfortable end with skill, but you also need that skill in graphic design to make an effective ad, persuasive writing, and a good idea of how to target the right audience. That perspective also costs time to learn from books and/or money if outsourced.