Wonders never cease.
Eurostar UK site is quoting UK154.50 as the least expensive ~USD300!!!
Eurostar US site is quoting USD138 - all in as far as I can tell.
RailEurope is quoting USD104 (after you take off the USD10 "protection option"). I didn't click through to "purchase" but this also appears to be all in, but maybe they add a delivery charge at the last step?