How much money do you make to be considered a success?

Wow, that is a lot 

Of course, I was waiting for somebody to say "you can't judge success purely in monetary terms". 

A really well respected text-book in an obscure field may sell very few copies - but it is still very successful and probably brought the author a lot of respect and fulfilment 

On a personal level, I know I won't make much money doing this - so the reviews on Amazon are very important to me.

What a writer makes basically depends on factors like how well they write, amount of interest there is in the subject, amount spent on advertising and promotion, ad nauseum.

The Bureau of Labor Statistics stated the median annual income for writers / authors in May 2020 was $67,120 [Pounds Sterling 48,501.31], as in half of writers make more and half make less.

Some truly great writers never earn diddly squat due to few if any sales, while others make decent money churning out less than inspiring work.

My mother once told me "nobody will ever read the crap" I write, so in my case success could be defined as people reading my work, enjoying it, and at times seeing the world a bit differently as a result.

Any money earned as a result is a bonus.

Quite right, Adrian!
As you and Cameron both alluded to, there is not only the thrill of creation and expression, whether shared or not, but there is also the gratification factor one gets when he learns something he created either entertained or edified or brought pleasure to someone else.

It feels GOOD to give the gift of pleasure. THAT can transcend a monetary value and in my mind, anyway, it means "success". When you learn that someone enjoyed your work enough to A) buy it; and, B) write a positive review, well, that is success consummated.

Vanity? Not always.

I consider myself fairly successful as an author because my royalties amount to about 500€ per year, and this is remarkable for didactic books the readership of which is always small.