I have not seen this in real life, but I think it's the most expensive beer ever made commercially.
Brewdog's End of History It was 55% ABV, only 11 were sold, and the bottles were stuffed into taxidermied roadkill. IIRC, the bottles were something like $2,000 a piece.
The most expensive beer I've had was Tactical Nuclear Penguin, also by BrewDog. I was at a class about beer at our local museum, and the presenter brought a bottle of it in. I got just a tiny sip of it, as the 12 oz bottle was shared between ~20 of us. At 32% abv it just tasted like whiskey. I believe after shipping the guy said he spent ~$100 on the bottle.
The most expensive beer I've ever purchased myself would probably be Black Flame, but I'm not sure. My local breweries' (White Flame Brewery, Hudsonville, MI) barrel aged Imperial Stout to celebrate their 1 year anniversary. They only made 120 of them, and it is the only bottled beer they have produced. That was $20. Luckily, they had it on tap on release day, so I can cellar the bottle I have.
If I think I'll like something, I won't think twice of spending $8-$15 on a single beer.