What price do you estimate would be "sufficiently low enough" given that a St Pancras to Folkestone Off Peak Day Return with a huge taxpayer subsidy is £50?
London-Calais day return tickets were cheaper than London-Dover ones (and without the normal peak restrictions of a cheap day return); I can't now remember the actual price, but - around a decade ago I guess - far far lower than £50 return, maybe half that. I take the point that selling cheap tickets to fill seats isn't financially sensible if if stops sale of the places to people who'd pay a lot more. But are there very many people trying to buy high-price
day return tickets at the last minute? It seems unlikely. Are there often vacant seats? On some days, presumably there are. [I got a Brussels/Amsterdam train one morning recently, at what I'd assume is peak time, but the train was at least half empty in Standard Class.]
Perhaps the way it would work would be based on a guaranteed return that day (ie, assuming you paid by card, your card would be stung for a large amount if the return ticket wasn't used), combined with very tight baggage restrictions, and a stand-by situation where you'd buy your ticket at StP in the morning without a guarantee you'd be on a particular train.
To answer the question, I'd pay a few tens of pounds for a last-minute on spec day return to Brussels or Paris. (Though I don't see the relevance of the subsidy on a domestic ticket that you refer to - assuming that's correct - to the pricing of a non-government-owned international ticket.)
