As the hidden treasure of Colchis, it is very valuable.
Though there is nothing particularly special about the fleece itself, for some reason there's a rumor that a dragon will appear if you throw it on the ground.
It seems that somewhere along the line, people mixed up the fleece with the dragon that used to guard it.
Honestly, it feels like some kind of satire.
At any rate, Caster doesn't have the ability to summon dragons in the first place, so it's all a moot point.
Oh, and there's a strange elemental dwelling within it, just like a certain Seventh Holy Scripture.