I like the idea! The UI is not too confusing, although I had to look things up once or twice, and perfect to get the job done. There's plenty of potential here - you could add regular bills, or requirements that are expensive to fulfill, or the option to save trees for later use, or eccentric clients with special requests that change if you take too long - "Uh, I know you're almost done, but I've changed my mind, I'd rather..." - something real clients never do of course...
Ok, ok, ich hör ja schon auf, mich einzumischen 😄