Just finished a WVR Desynth leveling session using Rainbow Sashes and wanted to post my results:

Starting Desynth Skill: 91.52
Ending Desynth Skill: 106.19
Total Desynth Skill Increase: 14.67

Rainbow Sash Quantity: 61
Rainbow Sash Cost: 555,588 gil

Gil to Desynth Skill Rate: 40,559 gil per skill level

Desynth returns (Market Board estimate): 141,149 gil

Gil to Desynth Skill Rate (Desynth Returns Included): 30,937 gil per skill level

I bought all of the Rainbow Sashes from the vendor in Foundation. Added 1 materia to every Rainbow Sash, starting success rate was 14% without Tinker's Bacon, 26.50% with HQ Tinker's Calm and NQ Bacon Broth.

Skillups ranged from .50 to .81. Was successful on 23 of 61 attempts.

Materials Returned

Battlecraft Demimateria x19
Rainbow Cloth x15
Rainbow Thread x6
Wind Crystal x23
Lightning Crystal x23