Peter IV (note that in the coin auction he is cited as Peter III as it was his ordinal as Count of Barcelona and other territories of Catalonia) introduced several numismatic innovations in the Crown of Aragon. Under no doubt, the most important one was the adoption of the florin -a coin that was widely used in Europe by that time- as the gold coin in general circulation throughout all the kingdom.
Pere IV began issuing florins in Perpignan, a city in current France. According to Crusafont seven issue marks (and not mint marks!) are known for the Perpignan florins by Peter IV: Catalan shield in lozenge, triangular shield, sword, crescent, rock, tower with pointed base and rosette of annulets. In addition, a coin with an S as issue mark was recently published. Out of them, the rock issue mark is one of the rarest: only two specimens are known and this is the only one in private hands. In fact, the specimen in the auction is the one represented by most of the catalogues.
The florins issued by the Crown of Aragon are not the most beautiful gold coins. They are typically not so well struck and their quality is almost never better than EF. For this reason, most international collectors are not attracted by them. However, this coin is extremely rare and its condition is the highest possible; further, it was minted in Perpignan, so it will be interesting for French and Spanish coin collectors, as well as for those who want to collect florins from different European countries.
Regarding the price, we can take as references other very rare florins in a similar quality. For example, Áureo & Calicó hammered in October 2011 a florin from Perpignan by Pere IV displaying a triangular shield as the issue mark (this coin is also unique in private hands). Although it was not as beautiful as the one now in the auction, it was hammered for its starting price: 3.500 euros. That same coin was hammered by the same auction house in March 2010 for 2.700 euros. We can also see that the coin with an S as issue mark was hammered last year for 2.600 euros. Hence, I would consider that 2.500 to 3.500 euros would be reasonable hammer prices for the coin in the auction.
Description from the seller
Pere III (1336-1387). Perpinyà. Florí. (Cru.V.S. 377) (Cru.Comas 11, mismo ejemplar, indica la existencia de sólo dos monedas: la de la Colección Vidal Quadras y la de la Banca Catalana) (Cru.C.G. 2200, mismo ejemplar). 3,48 g. Marca: roca. No figuraba en la Colección Caballero de las Yndias. Rarísima. MBC+.