Try booking way in advance. Funnily enough this afternoon, I checked the cost of a return ticket from Paddington to Carmarthen. If I booked the ticket 2 months in advance it was £28 for a return ticket - now that's pretty good.
However if I left it to the last minute it was over £100 IIRC.
Rail fares are now very expensive unless you can book ahead and fix the train you are travelling on and travel off peak. Unfortunately, off peak has become harder to get as the peak time is increased in duration to price passengers off the system at rush hours (both morning and evening peaks).
You will need to phone your train operator running the route in question (also not always easy to determine and may be more than one) and ask about the best advance purchase deals. The phone operators are normaly helpful and will try and find you the best price. To get these tickets, the dates and times of travel will be fixed and if you miss the train it is tough luck (just imagine Ryan Air on rails) and you will probably have to pay a full single to get home. Availability is also limited, so popular ones sell out early.
If you want a flexible ticket and to travel at peak times you will be paying full fare and it will be expensive.
It is also worth getting a rail card if you qualify in any way. A family card cuts the cost substantially if you are travelling with kids.