If you use bandwidth, minimum cost is not something you should be worried about at all.
I use over 500gb bandwidth daily atm and my total storage is just 117gb, so wasabi is out of question.
With b2 you'll be fine as long as cloudflare doesn't kick you, b2 -> cloudflare is free atm.
But after that your cost will all depend on how much bandwidth you use.
So, the cheapest thing you can do is buy a dedicated server from a decent network and use it as ftp storage, ovh or hetzner are good ones. Don't use hetzner if you plan on allowing adult content though.