I saw a site for sell on a forum. It was a nice site with almost 1,000 pages indexed, small traffic, like 300 daily visits, but best of all was a nice PR6 homepage. I quickly emailed the webmaster and asked him for a BIN, he said he had an offer for $450 but the buyer backed out, so I offered him that and he said he would email me later. So, a few hours later I get his email, where he says that he had just received a phone call from an interested buyer offering him $500 (I just think "phone call my ass, this guy wants more for his site"). So, I offer him $525 as long as he stops answering phone calls. He says he will email me back later. Ok, I was just eager to get this baby in my hands, I have never had a PR6, and that is the only thing I was interested in. He then emails me and says that his potential buyer has offered $550. That's when I started noticing the threads about PR update, so I decide to hang on and see what happens. Well, his site fell to PR5! Geez, if he hadn't been so greedy he could of sold it to me for $525, but I am not so interested in the site now, so I emailed him and sent an offer of $400. If he accepts, cool, if not, I couldn't care less, I was only interested in owning the site to have a PR6 in my portfolio.
For every story like that there is another person that paid big money for a PR6 and now has a PR4 a day later Its all about timing and with Googles erractic behaviour there is some luck involved too.
I had some similar, but opposite luck. I had a few inquiries on my software site which was PR0. Offered a BIN of $500, but had no takers. I figured it would do PR4/5 on the next update. To my surprise, the update offered a very generous PR6! Glad I held on to it!