Upgrading this way actually saves you a lot of gold, which is the reason why I'm doing it. It's about packing in experience for a 10 feed once the costs start to go up. If I fed 10 level 1 3-star necklaces to a necklace I'm trying to upgrade, it would give me (10 * 420) 4200 exp. If I instead fed 10 level 10 3-star necklaces it would give me (10 * 892) 8920 exp. If I were upgrading level 1 equipment, the first method would cost me (180 * 10) 1800 gold (so 4200/1800 = 2.333 exp per gold) while the second method would cost me ((180 + 180) * 10) 3600 gold (so 8920/3600 = 2.478 exp per gold). Even upgrading level 1 equipment this way is saving money. Upgrading higher level equipment costs more gold per upgrade material you put in so you'll end up saving even more money.

For example, these level 16 earrings cost 2124 coins per item to upgrade


If I were to feed 10 level 1 3-star earrings it would give me 4200 exp for (2124 * 10) 21240 gold (0.198 exp per gold), but if I instead fed 10 level 10 3-star earrings it would give me 8920 exp for ((180 + 2124) * 10) 23040 gold (.387 exp per gold).

For the first 10 feed, it's fine to just use level 1 accessories if you don't want to wait, since the gain is fairly small, but at higher levels feeding combined accessories seems much better.

-------------------------------------------------------------

Unrelated, but the other thing I've been doing lately is using my extra 3-star necklaces/bracelets to upgrade my extra 3-star earrings/rings (which gets them to level 8 instead of level 10 due to the lack of the 1.5x multiplier). The attack accessories seem to drop more rarely, so getting 2 of them to combine has been harder for me than just throwing one of my many extra 3-star necklaces/bracelets at them.