I appreciate the heads-up. I think I I’ll use barely any bandwidth (aside from originally uploading the assets to the S3 storage): there are just 2 of us and we’ll infrequently browse through our photos, like once or twice a month, and we’ll probably only view 50-200 images each time. Once PhotoStructure has sharing functionality, we might have 4-5 people viewing an “album” (50-200 pics from a recent vacation, say) one time, but that’s not a lot of bandwidth either. Even if there are some videos involved, they’ll be like a couple minutes long - maybe 10 minutes at the most. So I’ll definitely keep an eye on bandwidth use/cost, but I don’t think my use-case will cause it to be a problem.
While @Daniel is correct in that S3 is one of the most expensive storage APIs, I suspect the monthly cost would be less than a buck or two.
I’d personally look into B2.
Even when doing full rescans when upgrading to a newer PhotoStructure version? I think S3 bill on both bandwidth and number of API calls 
B2 is good - It’s within the price range that I’d consider reasonable for a storage product (~$5/TB/month) and many providers have pricing around that price point. You can often get storage VPSes around this price, but you’d have to also consider the price of backups plus the time to maintain another Linux box.
I haven’t tried running PhotoStructure on my server in production yet, but I have a VPS with 16GB RAM and 80GB NVMe storage, and another VPS with 10TB RAID60 HDD storage, both at the same provider in the same location (Los Angeles), and they communicate with each other over a private 10Gbps network. I’ve got a photos folder on the storage VPS, mounted via NFS on the other VPS.
I recently moved my backups (not PhotoStructure) to Wasabi from B2 - so far it’s been solid and might be worth a look.
Good point, I was only thinking about incremental syncs! I suspect you may be right.