The prices really depend. New releases are typically around $25 and their associated DVDs are usually about $16-18, so the premium is certainly larger than the difference in $/disc. Digital Copies are such a sham too. Most that I've seen expire after a year, so if you buy an older release, you can't even expect your code to work.
I'm not sure if resolution differences truly matter in the actual production costs though (ignoring the difference in price between pressing DVDs and BRs) since both require you to take the original source, which is definitely greater in resolution than 1080p, and resize/re-encode it.
I wonder if the lossless audio formats such as Dolby TrueHD help give the discs a premium in regard to licensing costs or processing time.
I prefer Blu-Rays, but sometimes the premium is just too high to warrant a purchase, so I just add it to my Netflix queue instead. The next movie I buy will be Inception next month (already pre-ordered on Amazon).