so many variables!
first off... what type of carpet are you looking for? wool? long lasting? something you're willing to replace in 5 years?
we have a local tradesman where we get all our carpets. he sells remnents. we've been able to get those for 50% of our carpet & lino needs.
for larger bits we have another carpet guy- who lays both the carpets he sells & the remnents.
when we carpetted 3 bedrooms (1 double, 1 single, 1 box) & hall/stairs/landing we paid 400 pounds. including carpet, new underlay, grips, wood ply on floor (have floorboards). we paid 245 for a better quality carpet in a medium size front room. but i must admit everyone within whom we've spoken says we got an AMAZING deal. (if you live in northampton/peterborough i'd be happy to give you his details)