Licensing is at the heart of this, many very basic functions that are free with Linux cost $ to have it on Windows. This is a reason beyond the moral to be a stickler about proper licensing.
A lot of people seem to be of the impression an independent IT guy is in the business of pirating software. I've had a few apparently disappointed people exclaim that if they wanted to (be properly licensed, expressed as "pay that much") they'd have gone with (the local big corporate IT services company)
But to the point, apotheon is right. I'd encourage a 'kitchen sink' approach to derive an initial number, eg start with a full license for bare-metal backups for every computer in the place, which of course is not necessary. But heck, comparing apples to apples if you run Linux you have that capability and more, for free.
When promoting Linux I don't (initially) shoot for a number based so much on what they need, rather I make a comparison of what they would have with Linux, and what the same capability would cost with Microsoft. Purely a psychological distinction, but sales are as much psych as fact. (most unfortunately)
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