This Great northern line goes from Moorgate to Finsbury Park in class 717s. The route is only 16 minutes.
In terms of pricing the general rule is £12 for a loco but I couldn’t see much value in a 16 minute route even at the usual reduction of £20 probably more looking like £15 and I don’t think DTG would see much value in building a route for £3pp most likely would see this suggestion more if it was part of another connection and this being like a branch or secondary line
I’d personally have recommended St Pancras International (TL), Kings Cross and Moorgate to Peterborough or Cambridge. Would have allowed for a great route on the ECML. Can have a 717, Class 91 and 800 for this route with layers of class 387 for GN services, 66 for freights, 700 for TL services from Kings Cross and St Pancras International. Then also we can have steam for Railtours.