I made a couple of observations while feeding new Spirit Weapons in regards to EXP gained when absorbing gems and items.
Basically, I wanted to kind of place a gold value on EXP an ego gains.
So I used a "control" test, basing my EXP gains on an NPC-bought item, in my case this was a 1,000 value "Potion Making Kit" you can get from about any Healer NPC.
Any lower value than that, even a 990 value Sasha Robe and the exp gained scales down as you go. (Sasha robe was worth 22 EXP)
I tested a few other items, but there is a significant case of diminishing returns for EXP gained from feeding non-gem/powder items.
At least this is the case for what I've seen, using only NPC-bought equipment. Anything over a purchase price of 1,000 seems to give the same EXP, whether it's a 10,000 set of Cores Ninja Gloves or a 470k set of Round Pauldron Armor, these items gave the same EXP as a Potion Making Kit, so I'm using that as the basis.
I tested the following Values on a 100/100 Bond Ego, for the 1.2x bonus feeding EXP.
Potion Making Kit - 24 EXP
10cm Spinel - 380 EXP
9cm Diamond - 4,189
10cm Diamond - 4,754
Now let's compare the exp:value ratios of these gems, basing their value on the relative cost you'd have spent if you had instead chose to feed the requisite amount of Potion Making Kits for the same amount of EXP.
For the sake of ease, "diamond tier" refers to Diamonds, as well as any of the newly added gems that came along with the Spirit Ascension Update.
10cm Spinel (or any non-diamond tier gem) relative cost = 15.833 Potion Making Kits, so a just a bit under 16k Gold.
9cm Diamond (or any non-preferred diamond tier gem) relative cost = 174.541 Potion Making Kits, so about 174.5k Gold
10cm Diamond relative cost = 198.083 Potion Making Kits, so about 198k Gold
I haven't tested preferred gems, but those should give more EXP than any other same-sized diamond-tier gem.
Hopefully this can make it easier for you to weight the costs of buying gems from other players for the sole purpose of EXP gain.
Also do note that if you choose to feed of 1,000 value items like Potion Making Kits, human players can receive a price discount when shopping at an allied race's NPCs, and to not forget about the Wednesday discount as well.