Solisplit is an application to share costs in a more solidaric way.
See about for an explanation on how it works.
There is a hosted instance at https://solisplit.soha.lt.
The easiest way to self host is using NixOS with flakes.
See example/flake.nix for an example deployment.
Alternantively you can also get an executable using nix build github:sohalt/solisplit, which you can start any way you like.