I've never found promo codes for Apple products - they just don't discount anything!
I did a few Apple photobooks last year and you are right they get very pricey very quickly - they also don't let you order everything together so if you have a couple of different photo books you have to order separately and get charged multiple times for delivery - £5.99 each time, which I think is a pretty poor show.
Having said all of that - the quality is great, and iPhoto just makes them so easy to create versus other sites, so I'm about to order a few more this year for Christmas!
If you find any codes let me know.