There is absolutely nothing in 5E rulebooks that give anything like an answer for you. So, to provide an answer, I'm going to have to go all the way over to Pathfinder, then draw some conclusions and make some guesses.
Tressym are a sentient species. They have an Int score of 11 (slightly smarter than a typical Commoner) and generally have the independent personalities of a cat.
So, let's set some baselines here. According to the Pathfinder SRD you can purchase a cat for 3cp...with the extra note of "You could probably just go find one if you felt like it." Tressym are far more rare and intelligent and valuable than a mundane cat, though. So that isn't very helpful.
The closest thing in rarity and value that I can find is the Pseudodragon...which is marked as having a sale price of 200gp. And there's a note in there that "given their intelligence, many consider the sale of a Pseudodragon to be slavery."
Looking at a simple comparison... In 5E and Pathfinder Armor and Weapons are roughly equivalent in price. Food is the same price. Adventuring gear is roughly competitive (some cheaper, some more expensive). In terms of animals, a basic riding horse is the same price as is a 'guard dog' (Mastiff in 5E). Camels and Elephants are wildly off in price, but most Trade Goods are also the same (Iron, Wheat, etc) Ultimately, it looks like for mundane things...prices between the two are roughly equivalent. Possibly a bit of flex here and there.
So, it's from a different system, but prices transfer reasonably well between Pathfinder and D&D...so I'd guess that 200gp is about the price range you're looking at. Just bear in mind that the 'breeder' you found is someone dealing in the sale of an intelligent race.
