Good points by others here already.
I think 5k would be the minimum to be honest. (but as others have said, we all have different experiences)
Also, at .99 you only make 20%. x6 5k stories = 30k, if someone bought all of them, you make a dollar.
x1 30k story at 2.99 makes you two dollars.
That was my thinking. HOWEVER, starting with short stories, could be a good way to improve one's writing without getting overburdened with longer lengths. Also 6 books = more shelf space vs the one maybe.
Also nothing to stop you taking those 6 stories and bundling them at 2.99. Cheaper for any reader who buys one and wants to read the rest + you make more commision too
Not sure I'm following the math, as I thought the royalty was 35% at Amazon, or maybe we're talking another venue? But that is only a side note. I'm replying to say that I, along with several others who post here regularly, have been guilty of thinking in terms of a single sale. No one's going to make a living that way. So let's say princephoenix finds his audience, they love his books and everyone buys most of them. On Amazon, he makes .35 per sale, or a couple of bucks on six stories. But, he's found his audience; 10,000 people buy those stories and now he's replaced a modest yearly income. And they don't stop selling, a few people buy his backlist as he writes six new stories. His audience grows. He's writing steadily, his stories are growing larger, along with his backlist. Boom! Full-time writer in 2-3 years, or seven months if he's determined. Impossible? Ask HfR.