Feats reduce the costs.
There is nothing in the rules supporting the concept that crafting an item your self has any changes on Wealth By Level. DMG 135 even tells you WBL is a basis for balance and suggests that you strictly control it and even the official Ask Wizard response
that even brings up crafting items for a profit gets swept under the rug and you're told the only reason the crafting Feats even exist are to explain how magic items are in game and how you should read the Magic Item Compendium for the updates on Magic Item availability.
To me, what you're doing really isn't any different than trying to push a case that the Fighter should be useful or that Toughness should be a great Feat. You want something useless to have some kind of value and feel that such a reason into it's self is it's own supporting point. Except I'm pretty sure I wouldn't have to listen to you drone on and on if we were talking about Toughness.
While a 20th level Artificer does have to craft the item at his actual class level, he's still reducing the price (plus who even crafts at that level? The campaign is over after a few more encounters).
Who said I was only looking at level 20? At level 2 any Spellcaster with Scribe Scroll can pay 12.5gp & 0.5xp to make a new Scroll but an Artificer must use CL2 has to spend 25gp & 1xp to make the same Scroll. By the 6th level when the Artificer gets Craft Wand (a wizard gets it at lvl 5 btw) if he were to make a Wand of each available Spell Level it'd cost him him 13,500gp & 540xp compared to any other spellcaster's 8,250gp & 330xp cost.
Even if cost-reducing Feats were houseruled to work they are not Artificer exclusive. Anyone can take them, and thanks to the MiC/PHBII an entire party can split the Feat tax on crafting your items, assuming the Arcanes just don't use Embrace/Shun The Dark Chaos, Gate, or some kind of Mental Pinnacle to Psychic Reformation to access and subsequently drop the required Feats when desired for maximum efficiency. Even free XP can be obtained by anyone willing to murder an Evil Humanoid on an Evil Alter which fits the well loved anti-hero cliche.
Tier 3: Trapfinding, Int based, dungeoneering class skills. Infusion list has access to tools for dealing with niche combat challenges or general buffs. Sounds like a decent support/skillmonkey to me. Spell-Storing Infusion is a perfectly valid tool as well, as access to ALL spell lists spontaneously is definitely a unique skill.
Capable of doing one thing quite well, while still being useful when that one thing is inappropriate, or capable of doing all things, but not as well as classes that specialize in that area. Occasionally has a mechanical ability that can solve an encounter, but this is relatively rare and easy to deal with..
Fixed your quote for you through you need to understand that it's still a pretty flimsy argument. A Samurai occasionally has the mechanical ability to win a combat-based Encounter but according to the post you pulled your quote from that doesn't prevent the Samurai from being listed as T6.
Likewise if you like we can stick to JK's dart thrown placement and still say the Artificer doesn't do one job decently by looking at the Classes that appear in the T3 & T4 lists. The Factotum & Beguiler both get the same Int-synergy for Search & Trapfinding except one has full 9ths & stealth and the other gets Inspiration & Int-to-Disable. They also get 6 Skill Points per level opposed to the Artificer's base of 4, but with UMD being absolutely required even if you ditch the required associated Crafting Skills the Artificer gets three Skills.
Dropping an entire rung to T4 the "skillmonkey" Artificer won't even pull the points the Rogue does and even through you may like his Spellstoring Infusion for being a throw-X-at-Encounters ominous win button, the Spelltheif's Steal Spell/SLA can be just as ominous, specially when casting acquired Spells into Spell Storing Arrows (infusions cannot be stored so no arty parallel), without requiring any XP investment. There is also the no PrC rule of being an Artificer. Since Item Creation and the Spell Storing Infusion explicitly use your Artificer Level opposed to your Caster Level any reliance on it also means your example of debate forgos nearly all possibility of PrCing out. So while the Spelltheif looks at the Chameleon & Hellbreaker for swapable crafting Feats & a better alternative of using stolen Supernatural Abilities to win. These T4'ers are more comparable to the Artificer than the outright superior T3ers.