Hello @M, RAKESH
Azure Storage Account is similar to Amazon S3 and is pretty cheap storage, pricing depends on couple of factors: Size of files, Storage Type, Operations and data transfer, Encryption.
I would recommend you yo use Azure Storage Account Blob Premium as price difference between Premium and Standard would be around 4$/month for 30GB in your case
Be aware if you create Azure Storage Account Premium you won't be able to convert an existing premium performance storage account to a block blob storage account with standard performance.
You can use Azure Pricing Calculator to predict costs.
Using Azure Storage Account Blob for 30GB backups with Premium storage type could cost you around 6$/month