To my understanding, the free AWS service are a relatively new thing. So just last year this has been a real, actual spending. Originally I found no information about the free AWS service, as all publicly available information indicates that this is still in ‘planned’ status. It’s hard to resonate on the budget when the exact terms are unknown. We can all just speculate at the moment.
I just wish OWG was more transparent about their S3 operations. Take example from https://hardware.openstreetmap.org/. Right now the public does not know what’s exactly being stored on the S3, nor what are the terms of the free AWS sponsorship.