I would agree on the deserving argument, I think Bow St came out with a decent BCR (benefit cost ratio) in the recent TraCC study, Carno less so, details on the sarpa website.
But wrt costs to reopen a long defunct station, next to a live rail line, surprisingly expensive as the Carno campaign group are finding. You have all the civil works, lighting, electrical, shelter, seating, help point, etc, etc. even for the most basic station. all done to NR spec & safety rules by large contractors with big overheads.
Dont get me wrong, I agree that Bow St and Carno should be re-opened. but in the case of Dyfi Junction, (which has the advantage of platforms on both branches), I think laying a bit of tarmac on the approach to a pre-existing station probably has quite a good BCR, in that it would generate more passenger journeys for minimal spend.