How do I validate my idea and make sure it's profitable?

So I built this small app, on the beta launch I got 30 user signups and about 20 subscribed to the newsletter, a month later I deployed a better version of the app with payments handling, now I am trying to see if people are ready to pay for this solution.

I am not expecting a lot of money from this app, I just want to make sure whether it's profitable or not, I cannot spend more time on development if I don't have the answers, I was thinking about running a Google ads campaign for few days and hopefully I'll be able to measure success of my app.

Posting on social media and all free medium generate random results, for example using the same content I can get 100 upvotes (shares) and good exposure today but not tomorrow, it's very strange. What you usually do to promote an early stage startup with a limited budget?