The core book seems to imply that a single cybernetic arm costs 15 Req, while locomotion (legs) also costs 15. Do legs come with a 'buy one, get one free' deal?

AFAIR, it has never been accurately stated or answered in FAQ, but as " legs " is written instead of "leg" I have always believed that this is the price per pair.

I always found the requisition of cybernetics a bit of a murky area. It makes no sense that gaining cybernetics and purchasing "The Flesh is Weak" are kept completely separate in crunch, since the point of the talent is that you willingly replace or upgrade a perfectly functional part of yourself with bionics (replacement of lost limbs is done automatically). Buying "The Flesh is Weak" should be a prerequisite for installing bionics outside of repairing battle damage (availability on advancement tables is not an issue, any Marine can buy the talent as an Elite Advance). The way it works right now, this is the only part of the game where you buy a permanent upgrade with temporary resources.

But I digress. The entry says "Bionic Locomotion", so it could be either a pair of legs, or a tank thread, or a set of wheels - anything, really. If you replace the means of locomotion, it costs 15 Req, period.

In my games, bionics are only available through replacement for battle damage or the allotted amount for techmarines. Astartes cannot simply "requsition" more and it doesn't fit with background. Apart from Iron Hands, and the Deathwatch isn't likely to appease them with expensive bionics just because.

If a battle-brother loses a limb, they'll get it replaced. They pay the requsition cost out of their next mission's allotment, but permanently keep the bionic. Given the restrictions and the utility of bionics, I think it's fair.