I am looking to travel from Barcelona-Sants to La Tor de Querol-Enveig/Latour-de-Carol in France, by train
I know that I will take the Rodalies de Catalunya's R3 line, which is part of Barcelona's network.
I can find the timetable for that service, but I can't find anywhere the fares for it.
The fares inside the 6 zones of Barcelona are findable, but outside those zones, I couldn't find either fares or how to get a ticket for my destination
As was already noted in a comment, for travel on Catalonia regional trains outside the Barcelona urban area, you can see the point-to-point fares by entering your origin and destination station on the timetable page or on the mobile application and picking any train. The prices don't depend on the day or time. The current single fare is 12€. A return is twice the price as a single. You can buy a 10-trip ticket (“Bonotren”) for a significantly reduced price, and it can be used by multiple passengers at the same time, though it seems that this is being discontinued and may not be available anymore. There are concessions for elderly and families (children above 4 pay full price unless a family discount apply) but I don't think a non-resident can easily get them.
You must buy a ticket before boarding the train. Barcelona Sants is served by both RENFE (the national train operator) and Rodalies de Catalunya (the regional operator), you need to use an orange Rodalies de Catalunya ticket machine or queue at the regional ticket window. You must buy the ticket on the day of travel.
If you board the train at a station without ticket sales, and only in that case, you can buy a ticket on board. Unless this has changed recently, Latour-de-Carol does not have Catalunya ticket sales, so when coming from France you'd buy the ticket from Latour-de-Carol to Barcelona on board the train. But from Barcelona, you must buy a ticket at the station.
Note that tickets must be validated before travel (similarly to e.g. France and Italy), although I'm not sure if that applies to single tickets — they may come pre-validated. In the main Barcelona station, you'll have to insert your ticket in the fare gates anyway, so you can't forget to validate.
I can confirm it is still (August '23) how it works
There are no sales of Rodalies de Catalunya tickets in Latour de Carol, you just hop on it and wait for the inspector to come around and pay €12 pp. in cash in exchange for a ticket printed on a paper slip
