The Hawkes also used to be rich before Leandra ran off, so the dog likely followed her. Malcolm also ran with an interesting crowd before he settled down with her, so he probably picked it up in his travels. Then again, I didn't even see the dog until after Hawke bought a home in Hightown, so unless there's something I'm missing, s/he likely got it then.
Besides, even pedigree dogs end up with "duds" in the litter, or sometimes manage to produce less valuable pups, or even mutt with other breeds (no matter how careful the handlers are, accidents happen). Just ask any dog breeder, or my sister who loves "rich people dogs" (like King Cavalier Spaniels or Corgis) which rarely make it to rescue shelters because they're so valuable. Those that do are generally ones with health problems or believed not to be as high or show quality as others of their breed. While Mabari are rich people dogs, I'm guessing the less valuable ones can trickle down into more common hands.
As in real life, the Kennel Master at Ostagar also implies that some dogs of the breed are higher quality than others. He mentions that the dog you rescue is a "promising member of the breed," and doesn't want to put him down for that reason. There's also imprinting; Mabari are war dogs, and thus likely imprint most often on nobles, knights, and soldiers. However, war has a high mortality rate (obviously), and when the owner does the Kennel Master mentions that "re-imprinting is very difficult." What do you do with a number of Mabari who've been bred to only obey their imprinted master, but their master is dead and they won't re-imprint very easily? Or (heaven forbid) become too injured or crippled to be effective war dogs from then on?
I get the impression that Cousland's mom gripes about it costing a lot of money because it happens to be a more valuable member of its breed. When you're the second most wealthy and powerful family next to the royal family, you're going to buy the best of the best, yes?