why do companies charge double for xlr cables?

check out analysis Plus same price rca's and xlr's

I think it's for one of two reasons. . .

First, the balanced configuration may actually warrant the added expense because an additional run of the signal wiring is needed. Some cables like the Kimber silver streak I believe, are configured single ended with a single run of silver for the hot side, plus a run of copper for the neutral (one silver/one copper). In the balanced version a second run of silver was added for the negative run (two silver/one copper). Add in (potentially) more expensive termination and you might have a reasonable justification for the doubling the price. I'm unfamiliar with Audience's cable construction but this could be the reason for the differential.

For the cables where the price difference is modest, the cable manufacturers may be sourcing bulk three conductor wire and then using it for both their single ended and balance configurations (essentially not using one of the conductors for their single ended versions). In this case there's not a material difference in their production cost. Or they are sourcing non-exotic bulk wire in both two- and three-conductor configurations and and again, there's not a significant cost differential.

Second, is that more people don't do what you've done (question the logic) and therefore the manufacturer get's away with doubling the price even if it's not justified by material costs.

XLR requires one more wire. Price varies on type of XLR connector. I have a 1m pair in house with $230 in just the connectors.

Only if your components are differentially balanced (DB)designs will XLR cables do much for you. That said, DB defines hi-end. Finally, we are seeing more and more manufacturers using DB.

XLR connectors are probably twice as expensive. The wire has an extra conductor and is shielded. It doesn't need to be fully balanced to improve contacts and reduce noise from EM/RF.