Ravens n' Pennies: Back to the Drawing Board
Mailanka's Musings: On the Cost of Advantages
Let's GURPS: "Fair Pricing"
I doubt it's possible to come up with a universal notion of what a point's worth and have it be meaningful. A point to a spellcaster using the standard magic system is another spell or a potentially valuable Perk (Reduced Footprint, for example,) to a fighter it's a Combat Perk, or it's 1/4 the way to another skill level in his primary skill, and so on. It varies, not only from character to character, but from group to group, and even from campaign to campaign with the same group. I think that means there's always a subjective element to any GURPS Advantage pricing.
My Example: Far-Caster
On my post about Magical Advantages, I listed Far-Caster at 30 points, which gives spells that normally take the Regular penalty for range (-1/yard) the Range penalty instead. This is an advantage I cribbed from an edition of Rolemaster (the one with all the Companions) that I wrote up for a campaign that started in 1995. I have been tweaking the cost of this advantage for literally 20 years, and I don't even know for sure that I've got the cost I want yet.
It started out that being a Threshold-caster was 25 points, and being a Far-Caster was another 25 points. At this point some Sorcerers were Far-Casters, but not all of them. After a couple of campaigns, it became clear to me that being a Threshold caster wasn't really much of an advantage over a FP based Mage I reduced the cost for Threshold casting to 10 points, then later eliminated it. I changed the cost for Far-Caster to 20 points, at which point all Threshold-users were Far-Casters. I increased the cost of Far-Caster back to 25, then to 30, and yet all Sorcerers are still Far-Casters. That's probably got more to do with a question of utility vs. raw power in this particular case, but I think it demonstrates that pricing something in GURPS is an art, not a science, and it's not always easy.
What About Balance?
I think that's another very subjective thing. The two important bits for me are that a well-made PC feels valuable and capable at least some of the time, and isn't casually overshadowed by another character who isn't focused on the same things. That can actually be an issue in GURPS; since you need a high IQ to play an effective Mage, a few points in social skills could accidentally make you better than the party face with at least some social stuff. There's obviously more to it than just raw skills, such as Charisma, Cultural Familarities, and other social Advantages for a start, but when a point in Diplomacy gets you a 12 without even trying hard there's a certain inherent problem with the balance.
For me that's not a really big issue, but I'd be lying if I tried to pretend it didn't exist. A GURPS GM really has to be careful of this sort of thing, and free with advice on how to spend points wisely even for experienced players. A social monster, properly built, will talk circles around Mr Diplomacy 12, and will be a valuable contributor to the group. Given the right situations, his points will feel well-spent and just as worth it as another spell for the mage.