If you want an honest answer, I'd probably be willing to pay a price equal to the physical book for a pdf. I consider the physical format to be in no way superior to electronic format, and indeed I find the electronic format superior in several ways. Since the electronic format has greater value to me than the physical book, paying the same price for a more valuable product is a good deal to me.
There's several conditions to that, however. The e-book needs to be DRM free. I want to be able to use any pdf reader I want to read it, on any operating system or electronic device I choose. I'm not particularly fond of Acrobat Reader, especially the linux version of it. I want to be free to use Evince or Xpdf or any other random pdf viewer I decide that I like without having to worry about whether or not the stupid DRM is going to work in the software.
I want to be able to make unlimited copies for my own personal use. If I decide I want a copy on my linux laptop, my old windows desktop, my modded xbox, along with several back up copies on CD, as well as any other computer or electronic device I buy in the future, I should be able to do so without hassle.
I also want my e-book available at the same time or before the physical book is. I'm not going to wait in order to get my hands on a pdf, and there's no way that I'm going to buy both the e-book and the physical book.
Finally, the e-book needs to properly leverage the benefits of the electronic medium. At the very least it needs to be well indexed and searchable. The more features of this nature included, the better.
To be honest, I'm not sure yet what formats I'll be purchasing for 4th edition(if I even purchase anything beyond the PHB). It sounds to me like the rules database portion of their online suite is more likely to be useful to me than pdfs would be. Pdfs would probably still be superior to physical books though, assuming they are DRM free. I don't yet have enough information to make an informed choice.