I don't know who the most expensive is; but you could always try going to some of the web hosting directories and reverse-sorting by price. That would get you a higher end, if not the most expensive. It's at least a starting point.
Several other threads around here have listed hosting directories, including the URLs.
I can't really be more exact than that. Different people have different concepts of "too high a price" for hosting. I've had more companies frighten me off, when I was in the market, because their prices looked too low for what they were offering, and I figured that there was some sort of catch. This was back in the days (1996-1997) before I knew a whole lot about bandwidth, resources, and how some web hosts market hosting packages to look attractive. Instinct made me take a step back.
*pauses, re-reads the dates*
Okay, now I feel ancient. Or just strange.