Just out of curiosity.  What's the most you have paid? What's the most you could imagine yourself paying for one? What would influence your decision?

65 dollars and that is it.  That is my cut off point for hard covers.

For a PDF, around $20. It depends on the content. I usually wait for sales. If I walk into the Sentry Box, if it's around $20ish dollars and really good (like when I saw DCC and on a similar occasion with BRP Gold Book) there's just no way I'm walking out without it. I think Adventures in Middle Earth was around $55, but I had to have it. I got Blue Rose yesterday. That cost me about $70 but that was a Kickstarter. I am more wary of Kickstarters as shipping costs from the US have skyrocketed. Oh, and Canada Customs thinks it's a fun idea to charge you a $10 fee for $1.72 worth of tax. The Strange probably cost me $90 including shipping. That was nuts. My upper limit is around $60ish unless it's something I really want. If you have book I want and it's a perfect bound functional book, then I'd probably pay a max of around $40ish. If you go over $50 it better be a pretty book. It also depends on how likely I am to use it.

I bought The World's Largest Dungeon for $80 (I think the retail was $100). I had more disposable income then and probably wouldn't repeat. Ended up trading it for some boxed set, I think either the Wilderlands of High Fantasy or the DCC setting.

These days probably $50. PDF, $10.

Assuming great content and production value, $60 for a hardcover book, $20 for a PDF.

In Canadian $75 max.

Considering the low volume of purchases I make, price is not really the issue. Even if it's $80, I can spring for the two or three must-haves of the year. The rest is just a few PDFs here and there.

Recently paid £42 for 5e Tome of Beasts. I think that's the most for a single book.

For a high quality 'campaign in a book' type product like Ptolus or Razor Coast I would happily pay £100+.

I bought the (two book) Guide to Glorantha for £125 and that is only a campaign world, but happily paid for it.

I'm one of those gamers who is time-poor but with disposable income. And a good campaign provides a lot of entertainment for your £/$.

ETA: What would influence my decision?

• Production values (sad but true...)
• Game system (if it's GURPS, I'll pay double. Seriously)
• Amount of game play it supports (ideally over 100 hours at the table)
• Structure (ideally sand-box with stuff for the players to interact with)
• Theme, which is tricky as the theme that attracts could vary over time

It depends. I'll go deep for something I'll get a lot of use out of, but I'll balk at more than $5 or $10 for a PDF I'm unsure of. I dropped (in USD) $100 on the two volumes of the Guide to Glorantha, and they're a great value from any angle. I could run games from those for rest of my days. Those books are huge; if they were printed at a normal size, 256 pages each, there would have been at least four volumes.

I picked up a copy of that Judges Guild Deluxe Collector's Edition that Goodman Games put out. It went for USD $100 on the Kickstarter, but I found a retailer in Canada offering it for CAD $90 (about USD $65). For great copies of three Judges Guild adventures, a few of their zines and a few articles, I'm happy with that price.

$10 for a PDF. I'm not interested in hardcopies.

For something I knew didn't suck? $200.

$50 for a hardcover. $20 for a pdf (but I would have to be in a goddamn good mood).

I bought the leather cover version of the classic Deadlands at GenCon one year from Pinnacle.  It is actual leather with the Deadlands logo branded into it, signed and it's the player's and GM's book all in one.  I think I paid $150 for it, maybe a little more.  Then I took it when I met Bruce Campbell and had him sign it too (he wrote the forward to the player's guide).

I have a few other collector's editions of things but that was the most expensive.

There have been a few PoD books that were in the $80 range, but they came with the pdf bundled in.

Normally $40 is a tipping point for me.  If it's more than that I need to have A.) played it myself and really enjoyed it, B.) have a group ready and willing to play it, in which case I don't mind being the one to fork out for it, or C.) read it and thought I could get enough value out of the setting material by being able to export chunks into other games.

I pre-ordered Ptolus, so I guess that is my historical limit.  I doubt I'd repeat the act on another book, and certainly not pre-order.  For something I want bad, I suppose I'd still go close to $100, but no one is producing that. The thing is, if a book has enough content to drive a hardback up over about 300-400 pages, I'd rather have it in two or more soft cover books.  There are hard backs that I wouldn't pay $50 for, but would happily pay $60 for three well-arranged soft covers with the same content.  Though in fairness, that's probably because I'd buy 1 or 2 of the soft covers and ignore the rest.  OTOH, I don't want 400 pages dribbled out over many $5 and $10 PDFs, either.

Consider the D&D 5E PHB.  It's a good deal, especially discounted online, and I'm glad it's available in hardback as an option.  But I would have quite happily paid $10 to $15 dollars more total for the option to get it in three soft covers:  One character creation, one magic, and one with the rest of the material.  It's be more useful at the table, especially if the sections/charts took advantage of the multiple front and back sections.