I did a quick back-of-a-fag-packet calculation for a 20W MD left on continuously for a month, and it costs about 65p. Mine switches on/off (OK, hibernates) when I arm/disarm the alarm, so it will be even less than that. Barely seems worth worrying about? Of course I am talking purely from a monetary point of view here, there is always the green argument.
However I appreciate it allows you to downsize the core, I have no idea how much juice my core is using.
The overall savings are in fact much greater especially if your system is larger. If you assume about 80-90p per MD, plus say 450p savings on downsized Core, plus less cooling/ventilation in the centralised cupboard/rack of say 120p & less wear & tear on the other equipment in the cupboard/rack too - say another 55p. Add all that up and you have a saving of just over 700p per month or approx £85 UKP per year (these are all based on real numbers from real installations).
Obviously all installations are unique and no two properties are the same but some simple incremental savings can be delivered easily by making the right choices. We have customers who are saving in the region of £185 - £225 UKP per year ($300 to $360 USD per year) by doing this. Small savings can quickly accumulate.
Obviously there are other incremental effects like being able to forget about ventilation when your media player only outputs 0.7w of heat, compactness, simplicity & very importantly reliability - which is a big one in fact. Reducing complexity by removing MD's is effective at improving reliability for sure. When we looked at reliability events the most common issues were related to MD's. So removing them not only saves energy costs (and purchase costs relating to the MD hardware too) but also improves overall system reliability too.
All the best