When added to a liquidity pool protocol, a jetton is defined by supply and demand. If you are looking for a way to sell a jetton at a single fixed price, then you're likely looking for an ICO smart contract:

https://github.com/ton-blockchain/token-contract/blob/main/ft/jetton-minter-ICO.fc#L56

Where the amount of TON sent with the transaction is multiplied by some value of jetton to mint to the sender.

int jetton_amount = buy_amount; ;; rate 1 jetton = 1 toncoin; multiply to price here

;; additional code

mint_tokens(sender_address, jetton_wallet_code, amount, master_msg);