An ultra niche is a niche within a niche. A minority within a smaller group. The outliers of the pack. The internet makes it possible to cater to such ultra-niches in a viable manner.
A piece of software that solves a particular problem for 100 people. Only 100. They would be ready to pay for the product since it is the only product or the best product that solves their exact problem. The amount they would be willing to pay depends on 2 things. First, how much of a pain the problem is for them. Secondly, how much value can be created for our users by solving this problem. What “potential” is being unlocked by solving this problem and how big of a “potential” is that. It can be in the form of freeing up their time or helping them do something more efficiently.
Finding the exact 100 people with this problem is the hard part. Maybe forums, reddit, fb groups, twitter all are ways to find people within a niche. Of which to filter again to find the 100 in our ultra-niche.