I would recommend making a distinction between Instantaneous and Permanent duration. The Djinni and PaO stuff is suseptable to Dispel and antimagic. Sure, permanent stuff could linger for generations in a low-magic area, but the means to get it would probably be lacking in low-magic areas as well.
Plant based stuff: Plant Growth spell is Instantaneous. The Enrichment use is fine for "fire and forget" if you just want to help a community and move on, but if you are looking for long-term mass production, stick to the Overgrowth on a small area. Mass produce resin, milk of the poppy, hemp, lumber, bamboo, whathaveyou.
Silk production: Improved Familiar from Complete Scoundrel, 4th level Arcane, Monstrous Spider. Vermin Keeper PrC can also do this at ECL 8.
Troll Byproducts: (or any creature with regeneration), combine with a ring of sustenance and some way to immobilize/paralyze, and then harvest away for leather, stomach acid for alchemy reagents, meat for dressmaking, blood for bloodmeal, bone for bonemeal, scrimshaw, scroll tubes, etc.. (check out cattle byproducts online for ideas).
Add on some "Half-creature" templates for more shenanegans. Half-dragon for immunity to fire/acid. Half-celestial for feather/fletching.
On a related note, I imagine Half-celestial sheep make mighty fine wool.
Edit: With respect to researching custom spells, I would recommend against it for this application. If I were the DM, and a player wanted to research a custom wall, but I knew the intention was to not use it as a wall, but instead as a means of converting it to wealth, I would disallow it. You might as well just ask to reasearch a spell called: Conjure Fifty Gold Pieces (along with the lesser and greater versions of it).
Also, I would LOVE to get some advice for how crystal based stuff can be conjured, what spells or powers that might use. Thanks!
I would also love to hear if anyone knows of any source material for crystal besides this teeny-tiny blurb in the ExpPsiHB:
Crystal, Mundane: Mundane crystal can be used in place of metal in weapons or armor, using a special forging process. The fortified crystal possesses the properties of a similar masterwork steel weapon or armor, except for visual appearance. Weapons and armor made of mundane crystal cost the same amount to make as their masterwork counterparts. Any item could potentially be made with mundane crystal
...If weapons and armor cost the same to make as their masterwork counterparts, then I would propose that the "special forging process" costs 1/3 the cost of masterwork.
Equipment cost, encounter treasure, wealth-by-level; these are all a means of maintaining game-balance. Sure, they can be broken left and right, but a DM can adapt and use infinite ways to re-establish balance.