I did a quick analysis of $/armour ratios for all types of soldiers and aliens. Reasoning was stats can be modded anyway and I couldn't really see any differences between a human soldier and an alien soldier (same stats available for both).
It seems the best armours are the Ion ones (sorry I'm used to the TFTD ufopaedia), with the flying one being marginally more expensive but giving the player freedom of space.
Muton/chameleon came in third with a good solid ratio.
Worst: Sectoid, then Human (no armour) then Floater but then the Floater floates
so it could still be a good choice when cash is scarce.
If anyone is interested in the actual numbers or how I did it (and critics are welcome) let me know.