There's also exceptions, such as Netherweave cloth which usually stays between 5-15g a stack. Netherweave should always be turned into something more useful, like bags to make even 4x more profit.
I find cloth farming the most fun in instances, but for the lower level cloths there are quite a few priceless instant respawn places outside the instances. Why I favor instances however is a high level player can pull the whole instance with one pull in most cases and then reset and repeat until the hourly cap is reached at which point it's a good idea to sell the loot and disenchant greenies and see if there's any other cloth that is valuable.Inventory is full
If you are clearing low level instances, you will bump into "Inventory is full" message sooner than you'd like. There is a cure however! I recommend Loot Filter or any other similar addon that automatically removes worthless loot from your inventory so your bags won't be full all the time. You should add worthless items manually to the addon's blacklist, and make it automatically delete grey items. You can also configure it to delete only items worth a certain amount or less.
Everytime you run to a vendor to empty bags, you lose valuable time you could spend killing things instead.
Note that higher level grey loot may sell for many gold pieces, so if you are farming level 55 -> monsters, you'll lose a lot if you ignore grey weapons for example.
Here's the best farming locations from my experience. Phased means you may not be able to farm there if you have or haven't done the quests involved. I haven't quested much in the below zones so there may be more than one phased farm. I've only included places with close to instant respawn mobs!
Linen Cloth
Ragefire Chasm (Instance)
Riverpaw Bandits (Westfall)
Risen Hillsbrad Peasants and Farmers (Hillsbrad Foothills, Phased)
Wool Cloth
Stormwind Stockades (Instance)
Shadowfang Keep (Instance)
Risen Hillsbrad Peasants and Farmers (Hillsbrad Foothills, Phased)
Ebon Slavehunter (Wetlands)
Dragonmaw Whelpstealer (Wetlands)
Silk Cloth
Scarlet Monastery: Cathedral (Instance)
Gnomeregan (Instance)
Slitherblade naga (Desolace, near shore)
Mageweave Cloth
Stratholme (Instance)
Scholomance (Instance)
Zul'Farrak (Instance)
Runecloth
Blackrock Depths (Instance)
Blackrock Spire (Instance)
Hellfire Ramparts (Instance)
Protip: In soviet Ramparts the mobs come to you!
Netherweave Cloth
Blood Furnace (Instance)
Mana-Tombs (Instance)
It's often more wise to farm other cloths instead, and if you are in need of Netherweave, buy some with the gold you obtained from selling more expensive cloths.
Frostweave Cloth
All WOTLK instances (Heroic and normal)
Frostweave Farming locations
Embersilk Cloth
Embersilk Farming locations
Did I miss your favourite farming spot? Let me know! :)