Having the forced auto-retreat cost you any tickets is a bad idea in my opinion. I say if auto retreat is forced, you should keep everything you have left. And then if you want, attack again with that force. It's only voluntary retreats that should cost you tickets and equipment.
Btw, what exactly is the formula for the cost of a retreat? I remember reading during the last strategus, that the dev team wanted to make it based on how long the battle went on. I originally thought it was just 50%, but that definitely wasn't the case with the Tulbuk defense.