I have shared my cost calculation spreadsheet (well, a slightly stripped-down branch of it, anyway) if you want to see what the best desynths are on your server for experience and demimateria. Follow this link for a Google Doc that calculates the cost of desynthing various items.

To use it, follow these instructions:
  1. Make a copy of the doc so you can edit it (File -> Make a Copy)
  2. On the "Desynth Skill" tab, enter your desynthesis skill in each class.
  3. On the "Raw Materials" tab, enter the price of the prices of each Raw Material on your server's MB in the MB field. Do not edit other fields on that page.
  4. If you want to add a new raw material, add it to the bottom and include the MB price or the crafting mats required as shown.
  5. On the "Desynth Items" tab, you can see the prices of desynthing each item, which tells you what's cheapest to use to level or get demimateria.
  6. To add a new item, enter the crafting materials as shown, the desynth class, the clvl (recipe level) of the item, and the number of materia slots it has.
  7. The "Desynth Chances" tab has the success chance table (unneeded since I now have the formula) and the scratch area for tracking exp gains.

Let me know if this helps or is confusing!