It's not just ticket prices, it's also the relatively infrequent train service from New Mills, and the lack of car parking at that station.
The frequency from New Mills is mostly the same as Marple these days Mon-Fri, but car parking is the major problem. Why even attempt that eye watering hill down to the station there when you can drive 5 minutes to Marple, get a space AND pay over a third less for your ticket each day.
One idea I'd like to see explored is to give Strines a much better service, at least hourly, and encourage people to stop there instead. There's plenty of space to expand the car park and ticket prices are only about 20p higher than Marple. If people knew they could actually get a train and a parking space there with much less hassle than the "Brabyns bunfight" of multiple car parks, it could have an effect.
For a very "wishful thinking" solution, how about a new New Mills station near their Co-op and the industrial units behind, which are level with the railway line and could provide a large area for parking. Or re-open the line to Hayfield (where I'm sure a lot of Marple patronage must also come from) and use that as the terminus for tram-trains...

Sadly Derbyshire and High Peak don't seem concerned whatsoever in the problems of ticket prices and parking there, never mind the effect it's having on neighbouring boroughs. A vast swathe of land next to Newtown station which could have been a Hazel Grove style park and ride is due to be an Aldi, with them giving a token gesture of about 10 overflow station parking spaces for the already useless car park.