BackBlaze B2 is probably the lowest price S3-like cloud provider and for 50TB (my current dataset size), it would be ~$250/mo: https://www.backblaze.com/b2/cloud-storage-pricing.html

Granted I didn't do this math before I dropped a few stacks on hardware. I really wanted to build it, configure it, play with it, etc. It's been a really fun project for me, and I've got a bunch of other stuff I want to do with it in the next few years (10GbE, X11 nodes, set up dedupe, etc).