The latest fad is 'designer breeds' such as *********, or labradoodles, for which breeders charge as much as �800, when in fact all you are getting is a crossbreed. Most pedigree breeds now start at about �500, and can be as much as �1000 - �2000 for difficult to breed dogs such as Bulldogs or Pugs.
Although Jack Russells are purebred, they are not a recognised pedigree breed as such, apart from the Parson Russell Terriers, and therefore cost less, usually in the range of �150 - �250.
I would always be wary of cheap puppies of any breed, as you need to be sure the breeders have carefully bred and reared the litter, having done all health checks and chosen a suitable stud dog, rather than using the dog up the road just because it is the same breed. It is also false economy to buy a dog cheap because it has no papers, even if you do not want to breed or show it. Reputable breeders would want their dogs to be registered and carry their kennel name for everyone to see.