Spell Rarity

[commongood]

Something's been gnawing at me... are prices on "Kunark level 55-60" spells based solely on desirability by players or does the rarity vary? I guess to keep it simple let's focus on just one level: level 60.

Let's say I kill a Cliff Golem and the RNG gods placed a level 60 spell on it... Is there a greater chance of that spell being Spell: Entrapping Roots than there is of it being Spell: Torpor?

Or is it simply /ran X where x=number if level 60 droppable Kunark spells?