Thank you to everyone who posted here. There's been quite a bit of interesting discussion.
A couple of observations:
- Unless you're playing in a published setting, supply and demand is so saga-dependent that it's difficult to meaningfully discuss. That is, supply and demand will be determined by the capabilities and desires of NPC magi. This is going to vary tremendously from saga to saga, so it's going to be difficult to say something that will be useful to another person's game.
- Various posters have quite different assumptions regarding the relative value/usefulness of various types of vis. I think this topic might deserve a thread in its own right, but I think it's also going to run into saga-dependent supply and demand problems. (Case in point - the disagreement here about the value of Vim. Likewise, I reach quite different conclusions about, say, the value of Creo than the article wolfenbach links to since I rate its usefulness/supply differently than the author.)
YMMV and all that, but I think it would be helpful to bracket these two factors.
The cost of an item/ritual/whatever is expenses+labor+profit. (Did I leave anything out?) Expenses are pretty straightforward - the vis used to make the thing. I'm not sure what a reasonable profit might be, but evaluating the cost of labor requires some way of comparing how valuable one magus' time is with regard to another's, which I find interesting.
That, of course, raises the question of how to measure the value of a magus' time. Vis seems the most obvious answer to me, but Berengar has suggested seasons of laboratory assistance, which I think is an interesting approach. However, given the social stigma attached to working as an assistant to another magus, is this method going to be applicable to magi who've been gauntleted for any length of time? (I realize, of course, that much - most, perhaps - trading would be in terms of barter, favors, etc. rather than vis or time. Granting that, I still think a discussion of value in terms of vis, time, etc. is valuable in that it helps one gauge how big a favor someone might expect in exchange for the ritual/item/whatever.)
Erik mentioned that it might be helpful to fashion a rule of thumb for the "ultimate wizard." Now, that would depend on a number of assumptions, but I'm guessing a good range would be from 80-120. (If Art1 is 40, Art2 is 50-100% of Art1, and various bonuses - MT, Int, aura, etc. - are somewhere from 20-40.) I think using the low end is probably the conservative option, as the others represent extreme specialists. You're reasonably likely to find an established magus in a Tribunal who specializes in one of the arts of interest, while finding a archmage specialist (Creo Imaginem with a focus in music, etc.) is a little less likely.
In order to simplify the discussion a bit more, I think it would also be helpful to quantify the range of effects a magus might reasonably purchase. My guess would be somewhere from levels 20-60. Anything below level 20 could be made by almost anyone. Serf's parma, but I recall that stat boosting rituals top out at level 60. Effects more powerful than that probably require a specialist, and are likely to attract attention. (You want a what? Sodalis, do the Quaesitors know you're trying to puchase a moon-duration castle-creating enchantment?)
The exception, of course, is longevity rituals in which magi will probably want the best ritual they can afford. That having been said, a magus can probably live one decade per magnitude of ritual (serf's parma). I forget where the "ultimate Ars aging analysis" is, but Final Twilight means that the law of diminishing returns should kick in big time around 16th-20th magnitude.
As for your situation, Tuura... Others may disagree, but I believe that PCs acting intelligently within the confines of game mechanics and the setting assumptions built into a saga should never be considered munchkinism. If the end result of a discussion of Hermetic economics is that 5 of your 8 magi can afford Ultimate Longevity Potions, well... YMMV, of course, but I vastly prefer that eventuality to a magus greying and aging while stoically repeating to his sodales, "This I do for game balance." (Please insert all the standard caveats about how it's your story and not mine and how you should do what works best for your story, etc. )