Eurotunnel prices vary quite a lot during the day. Cheapest outbound seem to be midafternoon and cheapest return late morning. A few years ago I found the CC discount to be very small, ok if you are already a member but won;t cover the annual sub.
If you get to Eurotunnel early for your booked train you may be offered an earlier crossing, sometimes at no extra charge. If you arrive late, I understand you are allowed on the next train 9so long as there is space) at the same price but thereafter have to pay more.
So I looked at Eurotunnel online and choose the latest cheapest crossing but arrive in time to catch either one or two trains earlier. I never had to wait for my 'booked' train. These were midweek bookings early September.
If the western ship routes are more convenient from your home base, Brittany now have an Economie service - no frills. If you make two or more return trips a year the Brittany Travel Club offers a good discount. You should break even in the first year and enjoy the discouts thereafter.