NumberOfCustomers * RevenuePerCustomerPerMonth >= MoneyINeedPerMonth
NumberOfCustomers * $3/customer/month >= 2000
Next you figure out: where the hell am I going to get 670 customers? I've been running one business for over 10 years and I think I have only 1500 customers in total. Finding customers isn't easy.
What I suggest is:
NumberOfCustomers = 10
MoneyINeedPerMonth = 2000
Now, no one is going to pay you $200/month for your TODO app as-is. But... What can you change to make it so useful that 10 people just throw money at you?
This is how I develop my ideas that are profitable from day one. There is a new software product I've recently created that I designed to make me $15K/day.
And it's working. Sometimes. But still, it's working, sometimes.
Edit: the worst thing you can possibly do is slap a price on what you've done. The reason is that you're just slapping a price on it, not because you've investigated the value added. If you really wanted to add a price of $3/month to people who are already using it, then add something that's useful to them. Perhaps integrate with Google Calendar as a pro feature. But now, you need to find 670 people who want your Google Calendar integration and need your particular TODO app. Not easy by any stretch.