Here's what you do for *easy* guaranteed Large Prismatic Shards.
Send to the tailor, make bolts of netherweave.
Tailor Netherweave Pants. 6x Bolts of Netherweave (36 cloth) + 1 Rune Thread (~40s).
Disenchant Netherweave Pants for mats: 2-3 Arcane Dust ~64%, 1 Greater Planar Essense ~33%, 1 Large Prismatic Shard ~3%
Tailor Arcanoweave Bracers (BoP drop from Arcatraz Sentinel): 6x Bolts of Netherweave (36 cloth) + 12 Arcane Dust + 2 Rune Thread (~40s).
Disenchant Arcanoweave Bracers for mats: 1 Large Prismatic Shard 100%.

To make 1 Arcanoweave Bracers, you'll need around 8 Netherweave Pants disenchants + 6 more bolts of netherweave: a total of 324 cloth + 10 rune thread. So, you'll be on average, guaranteed 1 Large Prismatic Shard and 2-3 Greater Planar Essence for ~4g of rune thread & 324 cloth.

Use the extra Greater Planar Essence to make Imbued Netherweave Bags after you scrounge some Netherweb Spider Silk.

That's a pure conversion of just netherweave into a large pris. shards for the cost of a few rune thread. Sell shards for profit, or keep for enchanting.

You'll have to do the math on what will get you the most money for the price based on your server's AH prices.. here's a comparison of the mats:
324 cloth + 4g = 1 Large Prismatic Shard + 2-3 Greater Planar Essence
1 Imbued Netherweave Bag = 72 cloth + 2 Netherweb Spider Silk + 1 Greater Planar Essence

The slick part of this conversion using the Arcanoweave Bracers is the final result is Greater Planar Essence and Large Prismatic Shards.. which both sell for high prices and make good stacks. Better than carrying around 50 stacks of cloth/bolts.