From May 2023 to July 2023 to/from Heathrow there have been an average of 2590 pax per month - that's arriving plus departing pax. Assuming numbers continue at this level, and rounding to the nearest pound, that gives a subsidy of £100 per passenger flying one way or £200 for a round trip.
An off-peak train fare from Dundee to London costs £ 96.40. There are typically 2 trains per hour from Dundee to each of Aberdeen and Edinburgh