PDFs Have Value
First let me say that PDFs, as you receive them, have value. The file you get is different from what a publisher sends to the printer, so they're not a "fire and forget" profit center for publishers. If you only buy the PDF (and not the print, if it's available), the design and development of the game you're purchasing is still worth money. Many PDFs have features that are very useful for the electronic format—hyperlinks, bookmarks, layering, etc.
PDFs Should Have Value
When PDFs lack some of the features that make them an advantageous format, they are more burdensome than useful.
At the very least, PDFs should have bookmarks. Without some way of quickly getting around it, PDFs take longer to use as a reference than a printed book. If a PDF doesn't have bookmarks, it's really bad when that PDF is locked. That keeps the end user from adding the much-needed feature themselves.
Speaking of locking, every PDF should be printable and copy-paste enabled. If it's not, it loses its most basic utility. This isn't a feature—it should be standard.
Speaking of features, the best PDFs are also hyperlinked throughout and layered to control their printer-friendliness. If this feature is beyond that capability of the publisher, a "printer friendly" version of the file should be included.
This is a complicated subject, so I won't tell you what I think is a good price—it's too subjective. I'll instead go over my purchasing habits, based on the pricing I see.
Print/PDF Bundle. On the surface, this should be a favorite option, but for me it causes some internal conflict, depending on the PDF-only price. One of the reasons I collect PDFs is to save space, but if a PDF is more than $15 or half the price of the print book (whichever is less), I'm very hesitant to make a purchase. The only way I will purchase a print/PDF bundle, is if I can get it at my FLGS, usually through Bits & Mortar, and even then only if it's in stock so I can peruse it. When print/PDF bundles are only available online, with no sort of cheap way to see the game—either by price or by quickstart—I'm much less likely to make a purchase unless the book in question is from a publisher I know well or it's a sourcebook for a game I already collect.
$20+. For some reason it's really tough for me to purchase a PDF at this price, with few exceptions. I really have to know what I'm getting into to hit "add to cart." This is not a value statement so much as insight into how my brain works.
$15. This is the top end of "sight-unseen" purchases for me. That said, a PDF needs to have reviews from trusted sources or just a ton of generally positive buzz for me to make the purchase.
$5. This is basically impulse buy territory for me. If I'm even mildly curious, I'm really likely to buy here. I've "discovered" many new go-to publishers this way.
Pay What You Want. For these PDFs, I generally plug in the next dollar amount above the listed average purchase price, if available. Otherwise, I plug in a completely subjective $1-$5. Rarely, I'll pay zero, and if it's something I like, I'll go back and pay as above.
Pay Now, Receive Price Off Print. This is a super-rare, but it's my really favorite price. In fact, I will often ignore my typical $20 limit. My guideline here is up to 2/3 the price of the print.
I've shared my thoughts and process. What are yours?