I'm aware that this topic has been dead for a while, but since no new efficiency graph has been included in the game yet, some more feedback might not be too late...
Anyway, with lots of challenges and high scores posted by now, we might have a better idea of how these different elements interact.
I've analyzed all the available data, and one thing stands out pretty clear, and that is that we cannot give cycles and footprint equal weight. It is much easier to improve the footprint than it is to improved the cycles, and we will have to apply some sort of factor, or else all "efficiency" records will be tilted towards minimum footprint (as can be seen by the challenge winners, who sometimes need thousands of cycles, just to shave off a few footprint counts).
After squeezing everything into a database! it seems that the best factor (which cycles and footprints on somewhat even level) is at 0.33
If we then add those two values (c/3+f), the resulting number will always give the lowest result for those solutions with a balanced approach.
In addition, if we throw the block count into the mix, we also reward those solutions that are truly minimal, rather than just a high tower squeezed into a small footprint. We can either add the block count or multiply it — overall the effect is only significant if somebody goes for a extreme (e.g. like Tiralmo, with his 4 block solution! in the first challenge).
Of course, I only had access to my own block counts to verify the validity, but overall the behavior seems pretty consistent.
So, this is what I came up with for now: 100000/((c/3+f)*b)
* c/3: cycles/3 (the factor discussed above, to bring cycles in level with footprints)
* f: footprint
* b: block count
* 10000/: just a convenient factor to bring the number down a bit, and to give more efficient solutions a higher number.
Since the block counts aren't easily accessible, I've added them to my InfiniSaver utility!, which also includes the efficiency calculation. The formula used in there can be modified by editing the .ini file.
Feel free to plug your own numbers into the spreadsheet (or tweak the formula in the spreadsheet or the utility), to see how it works with other numbers. It looks though, as if we might actually be able to pin down efficiency to a single, comparable number.