I don't have any hard and fast rule, but I would say 50% is a good guideline. As Mark pointed out, the companies setting PDF prices at the book price are doing so for reasons other than our reasons for buying the book/PDF, and I can't fault them for that.
Here are my real-world examples:
1. I paid $7.40 for Morrick Mansion in PDF from DriveThru, and that was before watermarks. But I did it because I was running the adventure in an online game (WebRPG), so it was handy to be able to cut and paste, even if this function was somewhat limited. I would have probably paid more for the PDF, since that is the format I wanted and needed.
2. I will probably buy the Book of Roguish Luck, but it is currently $9.00 in PDF on DriveThru. It comes out next month in book format, and Amazon has it listed for $13.59. This seems like a book I'll want to sit back and read, so I'll just wait for the book form rather than pay about 66% of the book price for a PDF.
That brings up another factor, that of time. Sometimes I am just in the mood to buy and enjoy an RPG book, and PDF has the benefit of instant gratification. The time difference is not as much if the book is available at my FLGS, but that probably means the price difference will be greater. The best value for my money is probably to be found ordering book format from someone online, but that means the longest wait to have to product in my hands. Of course, if I need an adventure to run tomorrow, the PDF format becomes an even better buy.
Echohawk has a good idea of paying a little extra and getting both the PDF and the book format. I think some companies have offered this, especially with print on demand.