£8 for the intro party isn't too bad, most of the clubs in town will be £5 or so at least
I think the union kind of stuff is fun to do when you first get there and are a fresher, its where most other people from halls will go, the union is a bit cramped and sweaty so don't expect to be able to have too many interesting conversations with new people while you are in there but if you want to have a dance its ok. Before long everybody becomes too "cool" for the union stuff and will start doing things in town!