Marc, Info about and purchase of the tourist Half Fare Card is here: https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/international-guests/swiss-halffare-card.html
CHF 120 for one month.
The annual Half Fare Card is designed for residents. You need a Swiss address to get one, they post it to you, and it renews automatically. Whichever version you have, prices are the some. You push the [Child/Half Fare] button on the ticket machine.
Please note you do not need to "book" Swiss train tickets in advance. You just buy them from the ticket machine (or human) at the station on the day. Unlike other railway companies there is no discount (above the Half Fare) for advance purchase and no reservations. A ticket is valid on any train on that day.
If you are flying in, you can buy your Half Fare Card and first ticket at the airport station.
The only reason to buy in advance is if you are arriving by train. If you buy your Half Fare Card and Half Fare ticket from the border on the SBB website, you then do not need to get off the train at the border station.