Why did Francois say Buck is worth 1,000 dollars?

From the text:

The general tone of the team picked up immediately. It recovered its old-time solidarity, and once more the dogs leaped as one dog in the traces. At the Rink Rapids two native huskies, Teek and Koona, were added; and the celerity with which Buck broke them in took away François’s breath.
“Nevaire such a dog as dat Buck!” he cried. “No, nevaire! Heem worth one t’ousan’ dollair, by Gar! Eh? Wot you say, Perrault?”

Celerity means swiftness or rapidity.