I'm having 2 living rooms, dining room, landing, hallway, 3 bedrooms (although one is very small) and 2 flights of stairs carpeted next week. All for £2,150. including underlay and fitting.
However the guy who is doing it was recomened to me by a mate who is a carpet wholesaler.
From what they both said, some modern synthetics are better than wool blends, but not much cheaper. Cheap synthetics are cr@p. They recommended going for a wool blend. There are different ratings 30,40,50, 70 etc and for domestic use they said for 40 was by far good enough, but we went for a 50.
Underlay is considerably cheaper off the net than shops. However if your getting a fitter in, let them fit gripper rods, they are quite particular where they go, and can alter how well the carpet fits.