If there was a single ticket price, regardless of whether the station or the terminal was the final destiion, then it would probably be the case that a ticket to Luton the town, which is the next stop, could be cheaper than a train to Parkway. Not sure if the rail authorities would accept that, as anyone starting a journey from Parkway would also be paying for a journey they did not make.
Network Rail will be building a separate passenger footbridge with 3 new lifts and 6 escalators as Parkway has 3 platforms serving its 4 tracks that will link to the raised Dart Station that will be level with the footbridge.