Hey there! I'm an undergrad so my experience may not be strictly relevant, but I'll try to help.
1) I had a quick dig on the website and managed to find this list of postgrad accommodation prices:
http://www.st-andrews.ac.uk/students/Ac ... dencefees/
As for comparing it to a private flat... to be honest it depends. I'm an undergrad living in a central flat (rent last year was £83/week) and I think that even including bills it's cheaper or, at worst, equals the cost of staying in New Hall self-catered or the David Russell Apartments. Central flats can easily be over £100/week (mine is 'cheap' because it has no living room) but will be much cheaper if further away from town.
To be honest accommodation around here is just generally quite expensive, including uiversity accommodation.
2) The policy for guests in New Hall (where I lived) is that they have to be signed in if they're staying the night, so that in the event of a big fire the emergency services will know to check that they got out safely etc. I'd imagine that the policy in other halls is the same.
Officially you can only sign in one guest, but a friend of mine in first year who had two guests over was advised (by a residence official who shall remain nameless) to get another resident to sign the second guest in, then have the two guests stay in her room anyway. So long as the sign-in book is accurate and you don't make too much noise, the residence managers probably won't care (and are unlikely to notice).
Bear in mind that the rules on guests etc. may differ for the David Russell Apartments, Fife Park and Albany Park as these are much more like individual residences rather than one big hall.
Hope that helped, best of luck.
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If Jack Bauer was put in a room with Joseph Stalin, Adolf Hitler and Nina Myers and handed a gun with two bullets, he'd shoot Nina twice.
If Jack Bauer was put in a room with Joseph Stalin, Adolf Hitler and Nina Myers and handed a gun with two bullets, he'd shoot Nina twice.