we are considering using Azure blob storage as storage for our backups. But we are not sure of what the transaction price would mean for us in reality. (they charge cost per storage volume and cost per transactions)
For example if I transfer one 16 GB file to the storage every day (and deleting so I in the end always keep 10 versions). Does that only mean 1 transaction per day (+ maybe a few for listing and such) or is a transaction like per packet of some size so that it will cost me loads each day. or what does the transaction mean?
Be careful, it may not be as simple as you think. Firstly, it depends on if you are using page or block blobs. It also depends on what library you are using to upload the blob. For block blobs, the storage client has a default value of the maximum size of the block being uploaded (32MB) and will split the file into n blocks – each block will be a transaction (see Understanding Block Blobs and Page Blobs. You will also need to consider retries, and as you point out, listing, deleting etc.
I suggest you look closely at how you are backing up and find the size of the blocks – then do the calculations. Then do some controlled trails in an isolated account and see if you can reconcile the billing transactions against your estimate.