Again, building software takes time and money. Much of open source has been "subsidized" by people either working on getting a degree, or by their employer when they build and release something that solves a problem for the company.

Kickstarters and other similar crowdfunding platforms are a great way to gauge interest in a concept before starting to build it. If thousands of people are willing to pay for it before it exists, then even more are likely to pay for the finished application.