My route that I would use SWUs is LHR/JFK. I have found cash J fares from £1000, and I've set my limit at £1400. Obviously to get such a fare it requires either a pandemic special sale fare, or an ex-EU type trip. I've averaged out the Y fare paid when I have used SWUs over the past few years and the difference between that average and £1200 is £650. So I value an SWU at £325. (Currently a typical J fare is £3500 which would value the SWU at nearly £1500, but I don't find that approach realistic).

Your own circumstances will for sure be different, but that is one way of doing it.