What is your money threshold to start a project?

When you plan a project as a business, one of the questions most people ask is what is the market size. I wonder, what is your threshold to actually start a project?

Imagine you know you could reach revenue of 3k/month with low operational costs and eventually low maintenance (but not 0). It's not something you could quit your job over, but nonetheless it is a reasonable stream of income. What if it would be 1k/month? Or if it would be 5k/month? What would be the threshold you set in order to pursue an idea?

Not everyone is aiming at unicorning, so I was wondering what goals do fellow indie hackers have.