My app is making $400 MRR with 0 ads. How much I can sell it for?

Why would you sell it? It's a great source of indefinitely-lasting non-earned income. Assuming no growth at all in sales of the app, that's 5 grand a year, 50 grand over 10 years.

I don't see the usefulness of fee trackers ending any time soon.

If you have one successful app, why not two? or even three? The world is your oyster.