Cost is the VPS instance, which I already had, and apple developer fee per year. I need the clipper for myself, and put it out there without any monetization strategy and many folks are using it now. Have thoughts about adding some features for monetization if users ask for more. So far, it is doing the job for me and haven't heard any burning asks from others, so plan to keep it as is.
Because we use it (and depend on it), I am vested in making sure it works and continues to work well, and doesn't get too complex or complicated, unnecessarily.
That being said, I've made some money from donations, grants, and people paying me to manage instances for them, for example.
It runs completely on the user's computer so there is no service to maintain.
It is a new kind of data management system that was originally an object store to replace conventional file systems; but the tagging features I designed made it useful for creating, querying, and analyzing relational tables.
It is a hobby, so I like seeing how much faster I can perform operations than regular RDBMSs. It is extremely flexible, so lately I have been testing it out using large data sets. Creating tables with 100,000 columns or doing a pivot table in a 227M row table is fun for me.
See my profile for links.
To keep costs down, I manage my own VPS and limit myself to projects that can run 100% client-side (e.g. no reliance on third-party APIs).
Shah Kur is a chess trainer that lets you set novel types of invisibility to help teach you to learn to play blindfold chess (without a board). It's got VAD + voice recognition (can use on your phone hands-free) alongside a WASM implementation of the chess engine, etc.
Lend Me Your Ears is a free piano game in the style of the old "Simon" toy which presents players with a sequence of musical notes and challenges them to reproduce the sequence using either an on-screen piano or a connected MIDI keyboard. It uses the Web MIDI API and YIN for realtime accurate detection of notes (so you can use a guitar for example).
That's just a few examples, but you'd be surprised how far you can get with nothing more than a client-side application.