Yes they're cross the breeds and BYBs and puppy farms are raking in thousands of pounds because people are prepared to paid £500 upwards for a "designer" dog which in reality is a mongrel.

All pups require the same care when they come into the world, and if that's done correctly, all pups should therefore 'cost' around the same.

Having said that, I think it's reasonable for a breeder who has spent a great deal of time and effort researching and showing her breed, to charge a little more than a shelter might for a pup they received as 'handed in' at say, 6 weeks. You are paying not just for the animal, but the breeder's time, effort, expertise, care, pre-conception health-tests, care of the pregnant mother, and hopefully too, the breeder's continued support throughout the dog's life.

I'm not so sure you'd get this same level of experience and support with a cross-breed 'designer' dog - it seems to me, breeders of 'designer' dogs are in it for the money primarily, not for the love and welfare of the dogs. THAT makes me very wary!

Perhaps if every puppy-buyer took heed of the advice given by the KC, by vets, trainers, behaviourists, and by the many resources on the 'net, we wouldn't have so many dogs in rescue shelters

Not sure of what was on the link but if someone is advertising something as designer dog I'm probably not going to buy from them. More than likely that is a mill or byb. If they have a bunch of different designer breeds it's a mill.
You are probably in for temperament & health problems. They will breed any dog numerous times over. No matter the poor genes & don't health test.
I randomly met a designer poo dog at the park, he had hypothyroidism & luxating patella. I'm sure the breeders only cared of the money they made.

I've had mostly pure bred but a few mixes too. Don't believe I've a need for pet insurance as it'd add up much more costly than vet bills. It is an unnecessary expense. I did look at pricing
I only looked at one insurance company. But pure bred were only a $1 more a month (for most med to large breeds). I wouldn't consider that much cheaper. The pure bred smaller breeds were the same cost as mix breeds. Even breeds like English Bulldog & Boxer were only $1 more.
The ones I found that were $6-7 more pure month ($72-84 a which is a notable difference!) were Saint Bernard, Caucasian Ovcharka (I was surprised), Boerboel, Tosa, English Mastiff, Kangal (wouldn't expect but I started seeing a trend), Great Dane, Neo, Bullmastiff it seems that the very large & giant breeds are more expensive. The large dogs like Dogs Argentino, Presa, Corso, Rott, Am Bulldog were only $1 though too.