No firm evidence of a premium price service but airport express normally means an excuse for a premium.
Passengers buying a return ticket from the terminal will have the Dart price included but passengers buying a return to Luton will surely be as now with the option to buy a ticket to Parkway or buy a ticket to the airport via the Dart. Many passengers just buy a ticket to Parkway in ignorance not knowing they can buy a through ticket as the airport is shown with a dotted line to it. I can't see how the same price can be charged for Parkway and the Airport unless the Dart was free but then how does the council get back its £225 million? If an additional price was built into any ticket to Parkway to cover the optional Dart then that would be unacceptable to any commuter who starts their journey from Parkway each day as they would be paying for the Dart and not using it.
The tap through oyster card should be working next year which will speed things up for many passengers.