Formal (and informal) hall is every night except Saturday I believe.
I've not been to hall for a few terms and never went that often, but I'm an exception and they're rather popular, especially Sunday formal hall.
The food is quite good and fairly cheap too. The main annoyance (for me at least) is the need to book in advance (by 11am on the day, or by Friday for weekend meals). Lunch and breakfast do not require booking though.
I can't remember exactly how much they are and can't seem to find the figures either...
My room is an A grade and the termly accommodation charge is £1,044.58, plus around £50 for the few days I'm back before term starts.
There's also around £15 of little charges like hardship/scholarship funds, carbon offsetting, events, etc.
If you get a scholarship or good exam/collection results, you may get cash prizes. I got £200 credited to last term's battel that way, and similar things in previous terms.