Eurostar is a special case. Even safeguarded rail staff who qualify for 'free' travel have to pay a fee there, I believe it's connected to the cost of doing the security checks.
€* has just joined the IR-scheme a year or 2 ago-before that they had some ''passholder fares'' which were more expensive as their cheapest advances. But then you did not have to use a ''travel day'' for it.
The high extra is for the TUNNEL charges, incl. also these checks. (as said-pressed on by the combined ferries of the time).
However, a coach fare-which may well be via the tunnel (though mostly ferry) f.e. AMS-LON is tipical 25/30€ if booked advance-always slightly cheaper as the lowest EZ airfare.
Sadly, more and more countries now have obligatory REServation for most IC-trips, like also on PKP in PL (though its often free), FI/VR, NSB/NO, CFR/RO. Some other countires do not really force you to, but have their own tickets sold with RES and do not indicate which seats are REServed-so its always a ''musical chairs'' lotto. Yes, it has been said so many times, FR and ES are the worst for it.
NBut-as a continental passholder I treated myself to 1st this time and thus spent 2 happy days on UK-trains, such as Virgin, LNER, GWR and got treated with 3 meals/day and lounge access.