$35 on a bottle of Angel's Share - Unique beer, have had better from Port for less though.
$30 on a bottle of La Folie - Haven't cracked this one yet.
$120 on a case of Fred from the Wood - I've still have 21 bottles left, excellent beer. Would have considered the $240 for the Cherry Adam if there was any left.
$70 for 8 bottles of Doggie Claws (2001-2008) - Completely worth it, turned it into a tasting evening with a few of the fellas and some nice stilton. Good notes.
I've spent $15-20 on single bottles on more than a few occasions. In many cases the beer didn't justify the cost, but the beers were rare enough or limited releases that made it worth the experience. The worst was probably a bottle of Jolly Pumpkin's La Roja that was one of the early batches (07 some time?) and was overly sour compared to later releases.
Honestly I'd take a bottle of Duchesse, Rodenbach, Unibroue, or Wisconsin Belgian Red over any of those for under $10 a piece.
As has been said, the cost per bottle is paltry in comparison to wine of the same caliber, and beer surprises me pleasantly more often. (and not only in the expensive bottles)