It is not the level of mob that determines the worth of the card, it's the rarity (and the attribute it gives to some degree). I seen a card that has description like "this card drops from a creature that only shows during a quest". Good luck getting that card - you only get one chance at it per toon. Some others with low spawn rate (I am looking at you, Myrmex queen) are also worth a lot.
Also, some of the low level cards are surprisingly rare. At the time your toon quests in that area, you have no housekeeper giving out free luck pots, you have no plussed wings, pets, treasure hunter rogue skill, etc. In other words, your luck bonus sucks. So no cards. And when you are high level, chances of low level mob dropping anything is lower, so you have little chance at the card.
Oh, and as far as farming low instances for music-making mats, that would not normally include bosses, so not likely to increase availability of the card.
As people mentioned, check AH history. You are only one that can check it, people without card can't.
-- Rustyx --- 92R / 92S / 92M on Reni (Guild KnightShift). Yes, running the new FOTM R/M, cause I am not elf enough to be WD/S.
Oh, and people who have more than 3 classes are clinically insane.