Not used the Santander crossing for about a decade, it was "expensive" back then, and more so now, but do agree compared to doing the extra miles to the Pyrenees from Calais the costs can be considered OK.
1,000+ extra tyre squaring miles of (relative) boredom is a factor to consider, ersonally I want to be riding the great roads ASAP and not spend long on motorways or dull straight 50mph limit rural French roads.
The cost of a Hotel on the way out / back, extra fuel and Tunnel crossing probably adds up to about £300, so if you can cross with a buddy sharing a cabin for £600 you are probably no worse off - although you might blow a fair wad of cash in the bar on a 20 hour crossing.