If I'm buying a PDF, then it's in addition to my hardcopy of the same book. In which case, I'll pay no more than 40% of cover price for it. The convenience of adding the PDF isn't worth any more than that to me. (Indeed, ideally I'd prefer to get it bundled with the hardcopy, as Paizo and MWP do at least with some buying options. However, I appreciate that not every company wants or is able to do this, and that's their prerogative - it's a "nice to have", nothing more.)
I do, however, expect it to be a PDF or similar standalone format - if I need to log-in to some dedicated service in order to access the material I've paid for, that's a deal-breaker. Nor do I want to be tied to a tablet, or indeed any specific device to use it. Every barrier they put on my unrestricted use cuts into the convenience factor, and it almost immediately ceases to be worth the hassle.
Unlike many other posters in this thread, though, I would be willing to sign-up to a subscription, probably up to $10 a month. However, this would be contingent on it including access to all the published materials - if there's a subscription and then an additional cost to access "book X", then that's again a deal-breaker. I don't mind renting access to the library, but object to double-dipping on the charges!