The bulk of the charges are per minute you are occupying HS1's track (For passengers). There is a electricity consumption charge as well, but I suspect the saving from running half the length of train will not offset having 400 less passengers to spread costs over.
Here's Eurostar's charges, for reference:
View attachment 170013
(Image showing a table of charges for Eurostar trains on HS1, based on the London-Paris service group).
Info is here:
https://highspeed1.co.uk/regulatory/access-new-operators
Eurotunnel charge a fee per train, and then a fee per passenger on top, IIRC. Their info is similarly available online as it's legally required to be. Both Infrastructures charge for having paths in the system, used or not, and have variable rates for short-term extras/changes and so forth as you'd expect.