In Sanctuary of Ice it says the Sinews of Knowledge would be willing to commission a Hermes' Portal to facilitate trade (p.74). Thing is, it's an annual ritual that needs to be cast twice a year (ArM5, p.156). I know it's a prominent Autumn covenant, but do they really have 30 pawn of Rego or Terram Vis a year to spend? Just wanting to know what's plausible regarding expenditures.
First of all keep in mind that Sanctuary of Ice is a 4th edition supplement, and I believe Hermes' Portal worked differently in 4th edition than it does in 5th edition.
I am not quite sure, but I think the 4th ed Hermes Portal was fairly similar to the 5th ed Mercere's Portal (HoH:TL p100-101) which is a permanent portal for which you only need to spend vis once.
Secondly, the tribunals of the Greater Alps do tend to be rich in vis. To be allowed to establish a covenant you need to show you have at least 10 pawns of vis per year for each magus in the covenant.
The description of the Sinews of Knowledge covenant (p65) says they have an annual income of 80 pawns of vis (mostly Corpus, but that can be exchanged for other types of vis) so they probably could spend 30 pawns of Rego/Terram vis per year if they wished.
ErikT is correct in that the Hermes' Portals of 3rd and 4th edition are almost identical to the Mercere Portals of 5th edition. The created portal is permanent and thus actually cheaper than a Mercere Portal at only 15 pawns of Vis. The only practical difference between 3rd and 4th is that the Portals in 3rd required a pawn of Vis each time they were used while there is no such cost in 4th.
Because of the similarity between 3rd/4th Hermes' and the 5th Mercere Portals, if you are using an older supplement then you should consider any Hermes Portals they list as ether a Mercere Portal or some similar type of Portal in 5th.
Yes, it was a one-off cost in 4th edition.