The Sporting Magazine (1792 - 1870) is considered the first general sporting magazine, though in its later decades it gave considerable emphasis to fox hunting. But it published all manner of sporting-related material, from news stories to poetry to calendars of upcoming sporting events. The subtitle of this magazine was "Monthly Calendar of the Transactions of the Turf, the Chace, and every other Diversion interesting to the Man of Pleasure, Enterprize and Spirit." [more at Wikipedia]
The December, 1821, issue featured a "State of the Horse Market etc. at the Repositories of the Metropolis," which was basically a list of various horse types and dog breeds and their current market value in guineas.
Newfoundland Dog, largest size ................. 10 to 20
Ten to twenty guineas in 1821 is roughly equal to $1,800 - $3,600 US dollars in 2021.