Heritage railways are expensive, most of them are fully a premium day out now. The NYMR is £45 for a round trip Pickering to Whitby which works out about 90p a mile, but for that you can make a full day of it which isn't bad for a day out, or you could ride up and down twice bringing that down to 45p per mile. Compare that to £69 for a day at Alton Towers (Adult walk-up) it compares favourably, look at it this way though, you have to meet the 21st century standards on many things, mountains of paperwork to plough through and factor in it costs something like £2000-£2500 just to get a Black 5 off shed these days, never mind the days coal, water and oil. 10000 gallon water tank is £800-1000 to fill these days I believe as well.
I agree in isolation heritage railways look expensive but if you then take a look at in the context of the tourism industry as a whole they aren't massively overpriced for a days entertainment.
If you like heritage railways, volunteer at one (one day a month would do) that offers the HRA card for about £20 a year you get free or reduced travel on 90% of the UKs heritage railways and get to contribute to the hobby, it really is a win win.