In theory, but of course cost is only half the equation. Reston serves a, ahem, limited population catchment. For the price of Reston station you could have built a hell of a lot of nice bus stops.
For the price of Reston you could have bought a fleet of executive coaches sufficient to run an hourly express service from Carlisle - Hawick - Selkirk - Gala - Melrose - Kelso - Coldstream - Berwick, and run it for a trial period of 5 years, and made it free for all passengers, and still have handed back £5m to the Scottish taxpayer.
I wonder if the fact that the Borders Railway and Alloa both significantly exceeded the expected number of passengers has meant people being over-optimistic about other Scottish (re-)openings.
They didn’t. They did in the first couple of years, because the assumptions for traffic build up were incorrect (as they are for most new railways and stations) but in the medium term they are much closer to the forecasts (Even allowing for Covid).
The rather difficult fact of the matter is that the Borders line was a political reopening, and wouldn’t get built now if it was proposed (The MSP responsible for championing the line freely told me this just after it opened). The rest of the old line has no chance, and rightly so. It would be a criminal waste of taxpayers money.
The only things which can underpin a national rail reopening are either housing or jobs - and since there aren't the latter in the borders, then it will be the former with the railway line providing the link to Edinburgh. But the locals won't want 10k houses which will justify the railway line's extension. In the same way the eastern section of EWR from Bedford can only be justified if it serves places like Cambourne (which is getting about 10k houses built).
EWR was originally predicated on well over 100k houses along the corridor. Many of those are already built - see the new states all around Aylesbury, Bicester, etc.
Stanstead was marketed as "London's 3rd airport").
Hence the engineers line reference (ELR) code for the Stansted Branch is ‘TLA’