I'm starting to run an Ars Magica 5th ed campaign set in the Novgorod Tribunal (well the tribunal was for the fourth ed but i like the setting because it's so different from my country) and i was wondering about the value of the vis and the books so the PCs can trade between themselves and the other covenants. On True Lineages are explained the exchange rate of the Redcaps but there is nothing about trades not involving third parties.
So i remember in older edition there was a table with the value of each vis compared to the vim vis but in the fifth ed there is nothing similar. I can't imagine a magus trading a corpus vis for an herbam vis... corpus can be used for longevity rituals and so it's more useful than herbam. But the True Lineages book suggests they have the same value. And what could be the value of an hermetic book compared to vis? (i.e.: if someone wants to pay a book with vis, how much should he pay?) Anyone can give me some advices?
The basics for running a covenant are spread over ArM5 (p. 218 in particular) and the Covenants book (all over). To this comes the trade via Redcaps in HoH:TL p. 84ff.
In general, first come decisions on the campaign and its focus, and only then exchange rates can be determined. How to generate prices of books you find on Covenants p. 94. General vis exchange rates are indeed best treated on HoH:TL p. 85 - but individual trade between covenants, eschewing the Redcaps, is fully troupe/SG decision.
In my Saga we use a rule of thumb that one season of a junior magus's time is worth three pawns of vis. This is about what one would expect to extract using a reasonable Lab Total.
So the value of books starts at 3 pawns for the kind of tractatus a junior magus could write. Summae cost about 3 pawns per 5 levels since they take about one season to write per 5 levels, typically. Similarly, lab texts start at about 1 pawn of vis per 30 levels of spells because a young magus can write about 90 levels of spells in a season.
From that base cost, you can add more if the book or lab text is high Level or Quality, if the magus who wrote it is famous, or (for lab texts) if the spells are rare or original.
As to exchange rates for vis, I would think Corpus and Vim vis are about as valuable as vis for the Techniques, and all the other Forms are roughly equivalent in value, but that's different from what the ArM5 books say.
That was i used in my last saga, but we decided that animal and herbam has less value because no one wanted to raise those arts in the covenant and corpus and vim value was on par with technique because of the longevity ritual.
I place Vim below those because of how any Magi can get new Vim Vis with a season of work, aside from that, something like that...
The basic advice, unless you want players to have easy access, default to more expensive rather than cheaper.
Single Vis for a low quality Tractatus, 5-15 Vis for good to exceptional quality.
For Summae you can start by taking Level+Quality, then anything from divide by 5 up to times 5, depending on how desirable, good or bad it is and how strongly the buyer wants it compared to the seller wanting to sell it.
Rubbish books can go at just a single Vis.
Vim vis can be distilled from an aura, but it takes effort, a season of work for 3-5 pawns isn't all that great a deal. A basic Aegis will take 4 pawns. Most magi will never get too much more than 5 pawns in a season, unless they are a Vim and/or Creo specialist.
Vim is, IMO, always in demand, because of its use in enchanted items, possibly longevity rituals, and the Aegis noted above.
I wouldn't say I put a premium on it, I think the premium simply is due to the utility. If a covenant does a Vim vis source, do they ever really have a surplus of that source?
And farming sucks. (Is my Jerbiton/Greater Alps prejudice showing?).
We run with the simple rule that Technique vis is worth twice of what Form vis is. I know some Forms seems more usable than others, and everyone can create Vim. But it is always a question of supply and demand, and we chose not to differentiate.
After all, Vim vis may be easy to distill, but will the magi spend this time? Will it damage tha aura as per RoP:Magic? And consider that a lot of Vim vis is also spend on Aegis of the Hearth and every time a magus opens a device for invested enchantments.
You live in the middle of a great forest and wade in Herbam vis up to your armpits, but no magus is interested in it - then itøs not very valuable. But your neighbours migth want it. And so on.
Also, assigning a 'vis value' for a season of work is a great idea, we also end around 3 pawns for a younger magus.