It's the market innit.
I always chose the tunnel rather than the ferry. A late booking a couple of years ago with a Land Rover with stuff on the roof at half term meant we were forced to use the ferry and it just reminded me why we prefer the tunnel. You'd think the ferry would be a more attractive option with the option to leave the car (or bike in your case) have a comfy seat and a brew etc but it just seemed slower and more disorganised. Given the choice of tunnel at £79 and ferry at £32, I personally would still go for the tunnel.