So the express-branded operation will be a premium-price service on half-hourly old trains that are actually bound for Corby, whereas Thameslink will continue with a much more frequent and cheaper service downstairs? I'll stick to Thameslink (some of the Thameslink trains only make one stop so are just as quick as EMR.)
The Dart cost will surely be built into the rail ticket price. Charging separately (twice) would negate any of the benefit.