I don't have my business off the ground yet but I've calculated that towards the beginning I'd need to make $43.50 per user per year ($3.65 per user per month) to break even, including my own cost of living.

Uh... guys, aren't you forgetting the number of users you have?

1 user x $100k

10 users x 10k

100 users x 1k

1,000 users x $100

10,000 users x $10

That's true - there are costs incurred that scale with the number of users you have and costs that don't. I guess the real way to answer this question is with a graph.