Thank you for your answer!
Well, it is true that nobody will ever summon a Greater Elemental with Planar Binding, Greater. However, I think we have to distinguish spells when they are normally cast, and spells when they are required for crafting something. There are many items whose required spell is only here because it shares something in common with the item. For instance, Mage Armor is required for the Robe of the Archmagi, however the robe grants a +5 bonus to AC, not a +4 as it should. Fox's Cunning, which grants a +4 bonus to Int, is required for the Headband of intellect +6. Resistance is required for the Cloak of resistance.
The book made sure players should use the appropriate Planar Binding spell for 1-18 HD Elementals when using the feat, but as they did not mention any other rule, it seems logical to me that the appropriate Planar Binding spell holds (the Greater one) for bigger Elementals. Anyway, if we assume that Gate becomes the required spell for 19+ HD Elementals, I am quite sure that there would be no XP cost. (Same thing, a normally cast spell should not be compared to a required spell, besides they never mentioned anywhere in the feat description that there would be an XP cost at some point). Do you think that what I just said make sense?
Also, thank you for the link. I've read it already (partially though) and I discovered the Magical Artisan feat thanks to it. This is a freaking good feat for an Artificer as it modifies the final cost instead of the base price. Sadly, it only works for one crafting feat at a time. If I were to play an Artificer, I would probably put it on the Craft Staff feat. Used with Apprentice, Extraordinary Artisan, Bind Elemental and of course the appropriate Craft Item feat, the gold cost almost equals one fifth of the initial base price...
Speaking of cost reduction, I think a player does not get the right to reduce the final gold cost by restricting the use of the item with a class or skill, as you implied he does on your posts. I think we have to distinguish items that can be created by the player, and items whose value can be assessed by the player here. We sure can assess the value of an Elf-only Headband of intellect +6, but the player isn't able/allowed to craft it. The crafting system is already cheesy enough, they don't need that. Same thing for the Skill reduction imho.
One last thing, you forgot to answer this: would you consider legal to bind an Elemental Monolith thanks to the Bind Elemental feat? Saving 25% instead of 20%. Thank you anyway for your past answer.