why do companies charge double for xlr cables?

Because they can is a good answer. Although a good XLR with gold pins is only slightly more expensive then some RCA's and considerabley less then the better ones. So this is not a good reason. The extra wire thing....well that's the manufacturers problem for not designing a balanced cable to begin with. Generally the shield is what is referred to here as the third wire, so any twin axial with shield can be used as a single ended cable or balanced. Discovery Cable has never charged one penny more for balanced cables and never will.
And for your second part, yes some electronics are not true balanced (phase correct and phase inverted) even though they have XLR's.

Discovery cable is fully balance as well and they will re terminate any of their cables to xlr for $100 They sound great to boot. Joseph runs a fair business- seemingly rare these days. I recommend him and his cabling!

so its a safe bet that if a cable company charges the same for XLR or RCA , that their XLR isn't really "fully" balanced? (Sry just trying to understand this better)

Yes, I would be stunned if any cable sold as a balanced cable was not truly balanced. I think people are getting confused due to the fact that many so-called balanced inputs and outputs on equipment are not balanced, and extending that assumption to cables.

A lot of the reason for the difference, IMO, is that in many cases the balanced cable is quite different from its namesake single ended cable, and in others it isn't.

So, for example if you use a 3 wire braid as a single ended cable the best sound comes from connecting two to the earth and one to the signal. For a balanced cable you connect one to each of hot, cold and ground (and putting more conductor on ground is not beneficial.

But in other designs I have seen the wire involved can be 50% more or 100% more in the balanced version of the cable. And the balanced connectors can cost 3 times as much in some high end lines. Whether this justifies a higher price to you depends on whether you are an economist, a cost accountant or conspiracist.