
£400 - £500 would be a fair price for a wellbred labrador, from a reputable breeder who will offer you well reared puppies from health tested parents, and offer you lifetime help and advice!
Those cheap pups are probably not from breeders of this calibre, and the chocolate ones if from the same source are likely overpiced poor ones.
You should expect to find that the breeder of a good litter can show you hip and eye certificates for both parents, and be obviously involved with some aspect of dogs, such as showing, Trialing, Obedience, Agility, or working Trials.