Yesterday I was asked by a client how much he could reduce the size of his content databases. I quickly googled to see if there was a query you could launch on the database to see how much space was consumed by the documents. I quickly stumbled upon a tool of metalogix which does that dirty work for you. It's called blobulator. It's not something you have to install on your server, just copy the files to one of your SharePoint server (there is a 2007 and a 2010 edition)
It gives you a nice table which you can then copy to your excel and calculate your overall gain.