How much would a weekend break for 2 in London cost?

Death.And.All.His.Friends asked: im planning on taking my girlfriend to london for a weekend get away together and was wondering:

how much it would cost for the two of us to spent 2 nights in a nice 3 – 4 star hotel in central london

how much it would be for sightseeing such as london [...]

Where can we go for a weekend to get scared senseless?

roskolewis asked: Myself and my girlfriend want to go somewhere haunted for a weekend break for a bit of an experience – any ideas?