Here's a simple set of calculators you can use to compare cloud storage costs between providers. The individual calculators also help you calculate price for a cloud storage provider you are already using or know you want to use.
Compare Amazon AWS S3 and Microsoft Azure
Compare Amazon AWS S3 and Google Cloud Storage
Compare Google Cloud Storage and Microsoft Azure
Compare Amazon AWS S3, Google Cloud Storage, and Microsoft Azure
Amazon AWS S3
Microsoft Azure
Google Cloud Storage
BackBlaze
IBM Cloud Storage
Oracle Storage Cloud Service