I saw a lot of threads showing some items that are quite cheap to craft, and disenchant in large prismatic shards, but I didn't find this one.
It's the cheapest way I found to get my shards, arcanoweave bracers are nice too but all in all the bracers cost you more than this if you buy the mats, or require more farming if you want to get the cloth by yourself.
Cogspinner Goggles : Cogspinner Goggles - Items - World of Warcraft
Of course, you need a friend with 340+ engineering, which is not hard to find at all. ^^
You can buy the schematic from those npc's for 6g, in Hellfire Peninsula:
Alliance: Honor Hold, from Lebowski.
Horde: Spinebreaker Post, from Mixie Farshot.
The materials required to craft those are the following:
- 4 Heavy Knothide Leather
- 2 Blood Garnet
- 8 Arcane Dust
For the leather, the cheapest way (without farming) is to buy regular Knothide Leather from the AH, and get a friend to transform them into Heavy Knothide Leather. (20 Knothide Leather = 4 Heavy Knothide Leather = around 5-6g on my server).
The blood garnets, well they are everywhere, you shouldn't have any trouble buying them for 5-6g per stack. (2 blood garnets = around 50-60s)
I usually farm some Netherweave Cloth for my Arcane Dusts (crafting Netherweave Belts and disenchanting them) to maximize the profit. Netherweave is so easy to get that it doesn't really require you to "farm" them, just go out and kill some mobs in Shadowmoon Valley. You'll usually need 3 stacks to obtain 8 arcane dusts, unless you looted some greens while farming the cloth, which you can disenchant.
Total cost for 1 Large Prismatic Shard (or a Void Crystal, but that's kinda rare):
10-12g depending on the prices on your server, but don't hesitate to use the trade channel to get the mats, I found out that many skinners don't bother reselling their leather on the AH and just NPC it for fast profit. I was lucky enough to get cheap and steady leather suppliers, which saves me extra gold (I pay roughly 8-10g to get one shard). If you find a steady netherweave supplier then you're good for making nice profit without moving your ass out of town.
Profit / day:
I don't flood the auction house with my shards, I've set my limit at 20 shards a day.
20 shards = ~200g in mats -> Selling the shards for ~32g (varies from 30 to 35) -> ~440g profit per day
Hope it helps.