These prices are from the Pendragon RPG. Like Ars Magica, Pendragon uses a pound currency, which it abbreviates as £. I personally find this much more convenient than writing "Mythic Pounds" all the time, so I also use this notation.
A Pendragon £ is pretty close to a Mythic Pound in its value: "the annual income of a typical peasant family." Because of this, the price lists for Pendragon align pretty nicely with prices in Ars. As a reminder, 1£ = 20 shillings = 240 pennies, abbreviated here as d for "denari".
There are two Price Lists shown here.
The Standard Price List is available in Dublin or Drogheda, which are both less than two day's ride from Uisneach, to the east. These items are in supply and can be bought within a day.
The Great City Price List is available once a year, at Tailten Fair. The Fair is held every Lammas (the first day of August). In addition, items from the Great City Price List can be ordered at Dublin or Drogheda (or through a factor or merchant) and can be expected to arrive a season later.
It is important to note that the prices listed here are for ordinary quality items. Fancy or impressive items should be Superior or Excellent quality, and grant die roll bonuses.
Superior Quality x2.
Excellent Quality x5.
There are some exceptions to this price list; Ars does detail the prices for some items which are on this list, and that trumps this list. So, for example, there is a highly developed book market in Mythic Europe and you know you can order pretty much any mundane book for 1£. That takes precedence over Pendragon's price for a book (which is "impossible to buy").