Based on what I've currently figured out, every item has a "multiplier" that they apply to items to determine how much XP they offer. Therefore, XP values between items are consistent between each individual item, so something that provides more XP to one item will provide more XP to another item, even if the exact amounts aren't the same.

I'm currently working on creating a list of components and their XP values, but first I have to decide on an item to use as a "baseline." Right now the Titanium Bangle is looking good since so far it makes the math work nice - the Gold Bangle earns 7/8ths the XP as it does, while the Rainbow Anklet earns an extra 1/8th. (In other words, the Gold Bangle gets -1/8th, while the Rainbow Anklet gets +1/8th.)

First, the quick 'n dirty explanation of how to quickly upgrade equipment with items available from the shop:

First, stock up on Mysterious Fluid from Creature Comforts. This is used to increase the multiplier. Next, stock up on Superconductors from Lenora's Garage. This item grants the most XP for the least gil, and should be used to apply the upgrades.

So, basically, dump 27 Mysterious Fluids onto whatever you're upgrading. This will give you a ×3 multiplier.

Next, dump as many Superconductors onto the item as required to reach the max level. (How much is that? Dunno, sorry.)

And that's it!

More will be added later...