How often do you the check prices before ordering in the bar? And if you do happen to ask for something that's particularly expensive, would you expect the barman to check that you're aware?
I have to say it's never been in my nature to check full prices of everything - and I have had staff check "do you know that's £xx" - but I might be changing my attitude after I got stung 28 Euros for a single beer last night! This was in Italy with table service so of course I'd already drunk it before seeing the bill!
To be fair there were plenty of price lists available that we hadn't seen, but the next most expensive beer was €10, and we had chosen from a chalkboard list that bundled it alongside bog standard Peroni & Tennent's at €4 each!