Blanket of Sustenance: This finely-woven blanket can be worn as a cloak by a humanoid creature, but is more commonly used as a saddle blanket for a steed. Once per day, it provides the benefits of the sustenance power, allowing the creature to go without food and water for the day. Faint psychometabolism; ML 3rd, Craft Universal Item, sustenance; Price 12,000 Cp.
It stands to reason that in Dark Sun a sustenance item would cost more than a ring of sustenance in a standard D&D setting. There is a slight error in description - this is a continual use item, not a once a day item. A once a day item would be 2400 cp. That would be breaking the spirit of the item design, however, because the sustenance power lasts for a day anyway.
While having a blanket item of this type is no problem, it makes more sense as a body slot item. I can imagine that merchant houses will invest in a collar of sustenance for their mekillots, who are carnivores and not cheap or easy to feed on the roads.
This price feels more in line with the feel of Dark Sun, as this would be 12k GP in other settings for balance purposes, but not needing food or water in Dark Sun is much more valuable than it is in other settings. Otherwise every other mid level and higher party would have these.
That is a good point, but as I’ve mentioned elsewhere, I would think genetic selection, or perhaps psionic selection, would see to it that sustenance and photosynthesis would become more and more common as survival powers out in the wild, until nearly all creatures would either have one of the two, or form a symbiotic relationship with a creature that does. Perhaps not for the city folks, where I would expect that economics would put the price of such valuable survival items at a premium, as you say. Templars might even confiscate and/or license such items in an effort to control who can come and go to their cities.