I agree with kreynolds except that I would add a body affinity cost (x2) since a shirt doesn't take up any space and there are few items that use the "shirt" slot. This is despite the fact that the Shirt slot has an affinity for Physical Improvement, which Enlarge certainly qualifies for. The fact is there are few adventurers with shirts, so it might as well be considered "not using a slot" which incurs a x2 cost modifier.
According to pg 214 DMG 3.5 "One shirt, vest, or vestment on the torso" is a valid slot, but imo rarely used by anyone.
While it may be a nice trick to get this effect all day, it would use a different formula (I think). I would use "Use-Activated or Continuous" which is SLxCLx2,000gp. So a shirt of enlargement would cost 1x1x2,000 and according to the footnote, since this is a minute per level duration spell the cost is multiplied by 2.
However, 4,000 gp is not the answer I would give my PCs, since the DMG urges me to compare it to existing magic items. I can see that Gauntlets of Ogre Power provide a +2 enhancement to Str for 4,000 without giving reach or other natural benefits of Large Size. However, gauntlets of ogre power also don't come with a -2 Dex penalty, and a -1 penalty to attack (which is -2 total for ranged attacks), not to mention a -1 to AC.
The fact remains that these magical items stack, so one could get +4 Str for a mere 8,000 gp by wearing the gauntlets and the shirt. I would raise the shirt's price by 2, as if it were not taking up a slot (it is taking a slot, but not a commonly used slot). In this way it would cost 8,000 gp to be enlarged all day (although I would rule that none of your equipment except items you are holding while putting on the shirt get enlarged. So other wondrous items, rings, etc - yes, but weapons and armor - no).
Looking to see if my shirt matches up with magic items of that cost (8,000gp) I see ioun stones that add +2 to an ability score are common, as well as some more obscure items. Its not as expensive as a Belt of Giant Strength (which would grant a +4 Str), but neither is it (strictly speaking) as powerful. Especially with the detriment that armor and weapons do not resize to their wielder, I think it would be balanced as a continous item at 8,000 gp.
I think a "Greater Shirt of Enlargement" could work by giving +2 Str, but none of the negatives, as well as allowing weapons and armor to resize, but it would be significantly more expensive.
So, to recap:
Shirt of Enlarge 1/day - 1 minute (10 rounds) 720 gp
Shirt of Enlargement - Continuous Use (does not enlarge weapons or armor) 8,000 gp
Technik