That really depends on a few things.
If they built it as something that they wanted to build anyway than having it pay for the hosting + some change might be a good outcome.
Also if your app is making a profit, even a small one then everything else being equal it will eventually make a return on those 300 hours if it holds.
I suppose if you're 20 years old with no responsibilities and you sit in a room coding 16 hours a day you could stretch $20k pretty far. If someone has the ability and desire to work for a formal $30k salary then I don't begrudge them declaring what they can do on the cheap.
On the other hand, I roll my eyes when someone who spent 500 hours building an app says it's profitable because they have enough paying users to pay the server costs.