The problem with charging big money for any working terrier is most genuine lads that graft there dogs can't afford to pay that sort of money. Especially when they need 5-6 terriers to keep up with there work load.
This means that Plummers go to people that want them for shows and to do abit of hedging. A lot of decent lads won't even consider a Plummer in there Kennel because of there price tag.
This has a knock on effect and that's why very few people work there plummers to ground.