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1 GB = 1,000 MB = 1,000,000 KB = 1,000,000,000 Bytes. So, 25 MB = 25,000 KB = 25,000,000 Bytes

Oh, Thanks!
I am used to thinking that 1GB = 1024 MB

Me too. I still believe yours is correct. 1gb = 1024mb but if approximated then it's fine.

From a currency point of view, having 1 KB be 1024 Bytes would cause all sorts of problems to users having to figure out exactly how many bytes to transfer to another user if they wanted to transfer ie 4 MB :)

The wallet allows the user to change the notation to Bytes, KBytes, MBytes and GBytes. Most users finds MByte to be the better choice leaving less zeros in amounts to transfer, and often less decimals to be necessary.

This is not quite right: 1 giga-bit = 1024 mega-bits, while 1 GB = 1,000 MB

Am I wrong?

And what's the usd worth of a GB please?

At the moment around USD 90.