That US$100k seems to have arrived over the last year, and their overheads - from quickly looking over their OpenCollective expenses tab - seem to be mostly buying hardware for CI things, and a monthly GSuite bill.
It's also pretty likely they get a lot of their resources for free / sponsored too.
It's also pretty likely they get a lot of their resources for free / sponsored too.
So, US$2k/mo might be fine (no idea).