If I cast a spell with an XP component using the metamagic feat Twin Spell (CAr pg. 84), will I have to pay the XP component twice as if I had cast the spell twice? Or is the XP cost unchanged?
The components of a twinned spell don't change
The metamagic is thematically described as casting a single spell twice, which might make you conclude you need to pay any component costs twice. However, the mechanical effect is that:
Casting a twinned spell causes the spell to take effect twice in the same area or on the same target simultaneously.
You are still only casting a single spell - it is the spell's effect, not the entire spell, that is duplicated. You're not actually casting it twice, and the feat doesn't say anywhere that you have to provide extra components when you modify the spell this way, so in terms of material or XP costs nothing changes.
The key sentence is the first one in the Twinned Spell feat description:
You can simultaneously cast a single spell twice.
Note that you are not casting a spell that takes effect twice, the spell is being cast twice. Therefore, the XP cost should be paid twice.
There appear to be a limited number of spells with an XP component that this metamagic feat could benefit, given the requirement to use a spell slot four levels higher than the spell's actual level and that both spells are cast on the same target. Atonement and Permanency have no benefit if repeated on the same target, especially as neither spell requires a saving throw. Planar Ally, Lesser is one of the few spells that might benefit from this, but it would rely on one of the called creatures being willing to wait until bargaining had been concluded with the other. (This GM would not allow for collective bargaining with multiple called creatures, but others might.)