Translation:This wine is really cheap. Maybe there is a mistake in the menu.
I wouldn't say using pretty as an adverb is colloquial. It is pretty common.
There's also a problem with words like pretty and rather: they're used so often in ironic understatement that they come to be understood as simple intensifiers. For me and speakers of my dialect there isn't much space between he's pretty ugly and he's very ugly. With somewhat and a little it's clear that it's understatement, but with rather and pretty they are more often used ironically than sincerely, so they they are understood as simple intensifiers.