Ffestiniog and Welsh Highland are expensive unless you are a member of one or both. Its excellent value for members at 10 free return tickets per year for £41 for an adult and 50% off the Welsh Highland Railway or £50 for the other way around.
Membership looks to be very good value, I assume you're provided with some kind of photo ID for the 10 free returns, otherwise it sounds like it could be open to abuse.