the reason the amulet that adds channeling is slightly more expencive is because it is harder to destroy being a part of your mage it will likely stay the entire battle (unless fighting a wizard who is directly targeting your mana)
the channeling enchantment (harmonize) also costs 6 to put on your mage for the same reason. but keep in mind you can add to your overall channeling (and a actions) by adding spawn points. for example, if your a beastmaster, and you have a lair out, yes the lair generates the 2 mana, not you, but when it summons a creature using its mana it is still reducing the cost for you. and casting harmonize on the lair is only 4 mana (cheaper than even a mana crystal)
so for example, the beastmaster can cast lair and harmonzise on turn 1. that gives the lair channeling 3 so you can cast 1 bitterwood fox per turn for 2 mana. and if you find one turn you need that extra mana, dont cast with the spawn point and the following turn you will have 6 mana available to cast something.
the same principle applies to all spawnpoints.