I would say Oxbridge > US Greek > other UK > US.
Oxbridge balls can cost over £200 if you also do the dinner, though most are nearer the £100 mark. They do both black and white tie balls, and most of the 38 colleges have a ball most years (white tie tends to be the triennial ball, not the annual ball), so if you are interested you can go to a lot of balls. The bigger colleges really splash out with up-and-coming bands(ie, the students have heard of them, even if the parents haven’t- Coldplay is an example), bumper cars and other fair-type activities, silent discos, fireworks, casinos, fancy booze arrangements, etc.
This is a story that the Daily Mail newspaper did about the triennial ball at Trinity College (a medium sized Oxford college) in June (with a little clip of the silent disco, which is always odd to watch!): http://www.dailymail.co.uk/news/article-4634994/Revellers-return-200-ticket-Oxford-college-ball.html.