Oxford is £7, but only for rail users, it's £31.50 for non-rail users. I don't know how they check that.
Edit: I do know how they check that now I remember - there's a display on the nearside platform, visible only from inside the gateline, with a code to discount the parking. However with that difference a return to the next stop to go in and get it would probably be worth it!
That's outrageous for Reading, there's really no justification for it being substantially more than MKC, and that's about 12 quid. Manchester Picc isn't really a place where many will want to park; Stockport is the "M60 Parkway" station and is £15.
Edinburgh I can kind of understand, as it's a very anti-car city.