50%, tops.
Let's do a little math here... it has been stated here on ENWorld several times that, in general, a publisher can expect to see about 25% of the MSRP of a book. For a $30 book, then, the publisher expects to see about $7.50... the rest is eaten up by retailers and distributors.
There are some costs germaine to both PDFs and print products (paying artists, editors, writers) so many costs are comparable (and have to be paid "in advance" of receiving money from sales) but there's one big cost that print products have that PDFs don't - the cost to actually print, bind, and ship the thing (the cost of bandwidth to transmit a PDF is negligible compared to the cost of shipping a physical book).
Even with rather large print runs, you're looking at an extra $2 (at least) per book eaten up in printing, binding, and shipping the things from the printer. That means your profits are essentally down to no more than $5.50 per copy (and remember, this is before accounting for paying the "talent" so it's not like companies are getting rich). Think of this as what the company needs to create the "intellectual property" which goes into the book they are selling.
Compare this with a PDF - most vendors will charge you 25-30% of the price in service fees, which means you see 70-75% of the purchase price. Now, to make the same $5.50 per copy (you still have to pay the "talent" et al) that you make in print after accounting for printing costs, you only have to charge... about $8. That's barely more than 25% of what the customer has to pay for the printed book... so in terms of maintaining a constant "profit per copy" the publisher really should only charge 25-30% of print MSRP.
Of course, Economics 101 tells us that the "right price" for something is completely unrelated to the cost of production, but is rather "what the market will bear." Sometimes "what the market will bear" is less than the cost of production... in which case the company is doomed to go out of business (many electronics manufacturers have hit this over the last few years). Sometimes "what the market will bear" is ludicrously high compared to cost of production and you'll make money hand over fist (for instance, soda companies and CD/record labels). This all comes down to "perceived value" on the part of the customer.
Now, it should be noted here that by and large the PDF market is an order of magnitude (or two) smaller than the market for print books, but the cost for writers, artists, editors, etc. is fixed. This means you need to make a larger profit per copy to meet that fixed cost. Thus, I as a customer can see bumping that 25-30% of MSRP to 50% of MSRP to cover those fixed costs... but you can't go much higher because there are many "discount" print vendors out there who won't be selling your product at MSRP.
I'll posit to you hree that most RPG PDF customers are savvy and are pretty familiar with the numbers above. They know that because the market is an order of magnitude smaller, companies need to make a larger profit per unit. They also perceive that they are actually bringing considerable value to the seller by purchasing in PDF in that they allow the seller to operate with much less cash expended up front. They ALSO don't perceive much extra value in the "features" touted by companies such as "Don't Take Up Shelf Space" and "Convenience of Cut/Paste" and "Index" and "Bookmarks" and "Replacement" - because (as we've seen) they know that such additions are trivial in terms of work and are basically inherent in the electronic medium. At the end of the day, these people seem to feel that they are paying for services - the writing/layout of "intellectual property" - and since that same "intellectual property" is contained in the printed form of the work, they will compare the price of the printed work to the price of the non-printed work, factoring in the "service" of printing and binding into the cost of the printed work.
In other words, the average PDF customer isn't impressed by all the "PDFs have more features" talk; they value the PDF based solely on the material - the writing - contained therein. They value the print product based on the writing contained therein plus the cost of printing/production. In other words, the perceived value for the print product is ALWAYs going to be higher than the perceived value of the PDF product. Which means when you're pricing your PDF, you have to be sensitive of the price of the print product... and not just the MSRP.
I'll take as an example (maybe bad) the Complete Adventurer by WotC, since there's information on it handy. With zero effort to find the "lowest price" I go to Amazon.com and find that I can order it for $19.77 even though the MSRP is $29.99 (we'll call them $20 and $30 respectively).
As a customer, then, I know that the price to me for "written material plus printing/binding" (i.e., the print product) is $20. Price your "written material sans printing binding" (i.e., the PDF product) at or higher than this price point, and you'll just wind up annoying me, because I perceive that I am receiving LESS (because I am not getting the additional service of printing/binding). I also know that an $8 PDF will make you the same amount of profit as you're getting from the print product sale. So as a customer I now expect the price of a PDF to be somewhere between $8 (where I know you're making the same profit) and $20 (where I can get the same product plus extra service).
When I factor in the price of "printing/binding my own print copy from a PDF" I expect to see a few bucks knocked off the top end of that range. When I factor in the smaller magnitude of sales a PDF generates, I expect to see the lower end of the price range jump up... let's say $4 each way, narrowing what I expect to pay for a PDF to $12 to $16. I could consider the extra utility cut/paste gives me, or the instant delivery I can't get from Amazon as things that add some small value to me and thus should up the lower end of the price range, say from $12 to $14. On the other hand, I can also consider the fact that you don't have to pay in advance to print my copy and warehouse it to be something that lowers the price, from $16 to $15.
Now we have a pretty tight price range - I expect - and feel it's fair - to pay $14 to $15 for a PDF version of a print product that has an MSRP of $30. Right around 50% of print MSRP, maybe slightly less. I feel the price ought to be about $8, as that gives the publisher the same profit (roughly) but because I am an intelligent consumer that knows how the marketplace works, I can allow your bumping it up as high as $15 as justified... but no higher.
So, I guess that was a LONG way of saying, 25% to 50% of a print MSRP, but anything higher is going to make many PDF purchasers say "unjustified high price."
And this is coming from an avowed PDF lover (I think my PDF habit has now passed the $3000 threshhold - coming close over the past 3-4 years to the amount I've spent on print products through my entire 20+ years of gaming).