For the past several years I have been using Amazon S3 to backup my WHM/cPanel accounts. It works quite good but the problem is Amazon’s complex pricing model (that charges you for things like diskspace, bandwidth, requests) making price for the service is quite unpredictable. I’ve read on many places that people often got surprised at the end of the month receiving the bill from Amazon.
Recently I have started testing DigitalOcean Spaces, a service similar to Amazon’s S3 (S3 compatible alternative) but with a pricing structure anyone can understand: for $5/month you get 250 GB of storage and 1 TB of outbound transfer (inbound transfer is free like on S3). If you need more than that it will cost you 2cents per every additional GB of storage and 1cent for every additional of outbound GB. Also first two months for every new space are free!