I've taken Amsterdam-London with Interrail many times, usually €35, sometimes €57, often sold out. So yeah, two price levels; Eurostar is basically SNCF.

I've had on multiple occasions Eurostar reservations cost more than 300 €. I didn't buy them, of course.

Those were normal reservations, not Interrail ones, which I explicitly was talking about, trying to get the conversation back on track to Interrail which is the subject of TFA. Those high prices are what I meant with "sold out" above; in that case you get the prices for normal reservations listed instead of the Interrail ones. They are usually marked as such, though, e.g. on RailEurope it'll say "Rail Pass doesn't apply for this trip". Pretty big difference.