I have been acquiring as many big box PC games as I could get my hands on in the last 3 years. I have built up quite a collection. the only prerequisite for me is that the game has the original box in decent condition and the original disc. I usually would prefer to have it complete with original disc case, booklets and brochures, but I sometimes find those items separate without the box, so it is always a priority to get the box first for me. I usually am willing to spend up to $10 on a boxed game in decent condition and up to about $5 for a 'not so good game'. If it is a near mint Sierra game or Lucasarts game, I could sometimes go up to $20 or even $40, but I have rarely ever paid that much for a single game.
Most of the time, I try to get a package deal and have done quite well in that department, raking in dozens of games for very low prices sometimes.
For games like Kings Quest I II III IV and other similar Sierra games in the original box with all manuals, I would pay up to $15
Flight simulator games with all original big manuals and keyboard overlays included is what I would consider spending the most on. I paid $40 for a copy of Falcon 4.0 with all the manuals and maps and the original binder and original CD. I think that was one of the most expensive purchases I made for a pc game. It was more than worth it and I would do it again for Falcon Gold with the original VHS tape includes (Art of the Kill)...I may even be willing to spend up to $60 if everything is mint for games like these.
For all Lucasarts games that are more common, I usually buy them even if I own them already if they are a good deal. For example, I have 3 boxed copies of XWing collectors CD edition because I bought them so cheap. Other more rare Lucasarts stuff like Indiana Jones, Monkey Island 1 and 2, Maniac Mansion, Day of the Tentacle, Full Throttle, etc...are very difficult for me to find and I think I may have to bite the bullet one day soon and just splurge to get them to complete my collection. I see those games going for over $60-$100 on eBay (including the shipping to where I am)...I have a feeling you lucky ones in the US can buy this sort of stuff for much cheaper...I am jealous. I was lucky recently in that I got a copy of Sam & Max Hit the Road for only $5 in the original box with all manuals and documentation. I do find stuff and I actually have some gems in my collection, but I tend to find things in lots and it is over a LONG period of time it seems and it takes A LOT of looking on craigslist and other online classifieds.