Licenses isn't a hardware discussion. So that's moot.
The hardware, you're not going to buy 1/4 the hardware if you go Linux, you're almost certainly going to buy the same hardware, but perhaps get more out of it. In any event, an "average" server, the cost to go from say 64GB to 128GB as a host, is 'marginal' in the grand scheme of all this, especially when factored over 3 years.
Also, this solution if I recall was for 5-10 users. While Linux solutions will certainly run on less, there comes a point where there is a "minimum" size. An Exchange solution for that number of users, is going to run just fine on your average laptop, as long as it has an SSD. So to say that it would require 4x the hardware, is absurd. It's going to take maybe 2-4 cores and 8-12GB of RAM of a host, and that's if you're being generous.
Thus, the hardware cost can be assumed to be a wash.