I've managed to avoid the Audible 50% off trap this year. I've had enough credits of late to keep me in books, and when I don't, I've hit enough WhisperSync deals that the 50% doesn't seem like such a great deal.
Overall, I've managed to keep my total eBook costs to a number I can live with this year. But the enforced tracking I've imposed on myself has made me much more aware of costs, and I expect next year's book costs to go down some more. Especially since my TBR is up again this year.