Thanks for the responses, in many ways I think the the answer to this depends on how you want your Glorantha to be. But to continue the debate some more 😉
Under Functions of a Temple - Gain access to Rune spells (Pg 284) it says: "When an adventurer sacrifices a point of characteristic POW at the temple, the adventurer may gain access to a special cult Rune spell available to that temple, including associate cults." I understand that to mean that not all temples have access to all Rune spells, Shrines have access to one special rune spell, minor temples to all special rune spells, major temples adds all common rune spells.
How you define "access to" is key here. I am defining it as access to but not automatic learning of the magic. To learn rune magic I think you have to:
sacrifice your points to get personal access and
be at a holy site that gives you access to the specific rune magic.
This also has the added benefit of giving players a reason to travel to a different holy place if they want to get access to a specific piece of rune magic. And who knows what happens on the way there.
In the Vasana example on page 314 she is at the tribal temple (a major one). However if she was at the clan one she may not have had such a wide range of options.
None of this addresses the 100L cost point which I guess is YGMV thing, indeed all of this is 😀