A friend's site, where users upload lots of pictures every hour, gets millions of hits everyday. It costs him $20-$30 on S3 per month.

You're probably overthinking this. Just run the test. Chances are no one will care about your app.

But if you get lucky and get a bazillion hits, you'll find out very quickly. Stick some Google ads on. If you capture emails, you should also be able to convert some to premium.