It looks like up for running something like express/koa/etc while fx is for functions (like AWS Lambda). They are both "serverless" but that's about all they share in common AFAICT. Thanks for mentioning "up" though, I'd never heard of it before, very cool.
Yep it uses AWS Lambda currently, you'll usually end up paying < $1 for small side projects such as https://gh-polls.com/ etc, but it definitely depends on your traffic.
This project looks like it's more FaaS style (sans the self-healing / autoscaling etc). Up is more application oriented, it just happens to use Lambda.