Yeah...
I'm gonna say no. If you want the technical argument, these feats both say you pay 75% of the normal cost, not that you reduce the normal cost by 25%. In other words they do the same thing, so they don't stack. Using Sudden Empower on a spell that is already empowered with the Empower Spell feat wouldn't add another 50% damage (at least not in my book).
Second of all, you are clearly reading the feat wrong. You appear to assume that the feat allows you to craft a 1000 gp item for 250 gp. What it does is reduce the 500 gp creation cost for a 1000 gp item to 75% of 500 gp, or 375 gp in this case.
Saw this thread browsing google for optimal item creation and such. even though this has been dead for 13 years, I'm posting this for any future people searching for Artificer optimization.
the Artisan Feats do indeed stack, but in a unique way. you cant take Extraordinary Artisan (or any of the other Ebberon artisan feats) multiple times as it was given an errata before 3.5e ended. How ever Legendary Artisan, Extraordinary Artisan, and Exceptional Artisan are all item creation Feats.
They themselves let you qualify for Magical Artisan
Magical Artisan is a General feat, and can not stack with itself.
As Steve Jung pointed out and showed from 3.0 to 3.5 the feat was re-written to now say.
Magical Artisan [General]
Prerequisite: Any item creation feat.
Benefit: Choose one item creation feat that you possess. When you make an item with that feat, you only pay 75% of the normal cost to create the item.
Special: You may take Magical Artisan multiple times. Each time you take the feat, it applies to a new item creation feat.
and for example Extraordinary Artisan says
Extraordinary Artisan [Item Creation]
( Eberron Campaign Setting, p. 53)
You are an expert at creating magic items at a lower cost than usual.
Prerequisite: Any item creation feat,
Benefit: When determining the gold piece cost in raw materials you need to craft any item, reduce the base price by 25%.
they are different effects coming from two different sources. In the case of Extraordinary Artisan you reduce base cost. Meaning for every 1000 gp you only pay 750 gp in cost. this lowered cost becomes the new base cost of the item. Magical Artisan tagged to Extraordinary Artisan means you Now pay 75% of the base cost, That 75% is of the new base cost.
so as an example Kyton Armor from the MiC pg 19 cost a total of 13,100 to buy.
the Cost to Create: 6,000 gp (plus 1,100 gp for mithral shirt) the base cost is the 6000 GP Extraordinary Artisan lowers the base cost by 1,500 (25%) leaving you with 4500 as the new base cost. Magical Artisan says you pay 75% of the base cost. that is 75% for 4,500 or 3,375 gp, then you add the cost of the 1,100 for the mithral shirt so over all you pay 4,475 instead of the 7,100. used this way the two feats cut the gold cost of a Kyton Armor by 2,625 gold. Its not that big of a reduction but it saves 39.97% of your gold.
Another way to think of this is as a math problem with the following equation
(Original Cost-(Original Cost* .25)) * .75 = total
(6000-(6000* .25)) * .75 = 3375
using an equation calculator or Microsoft Excel you can program the equation to look like this N represents the new cost.
Original Cost =𝑐 ;(𝑐−𝑐(0.25))(0.75)=𝑛;
Because Magical Artisan can be counted for each Item Creation feat it can be taken more then once and the most optimized outcome involves taking it with each of the other Artisan feats, this normally would be a 7 feat investment: initial Item creation feat (like scribe scroll) the ebberon artisan feats followed by magical Artisan.
It is plausible however; for an artificer to pull this off, considering the other artisan feats are all bonus feats. Artificers can gain Legendary Artisan, Extraordinary Artisan, and Exceptional Artisan as early as 4th, 8th, 12th level with bonus feats; Then take Magical Artisan 3 times at 6th, 9th, and 12th level. Doing so means after 12th level with the exception of forge Ring an artificer can reduce the Gold, XP, and Time of all other magic items they create by about 39% (minimum one day for time.)