Token creators don't define any price. They do define things that influence the price like initial supply, inflation, etc. The market price will emerge from the interaction between those things and any demand that might exist for the token for whatever reason (usually just speculation). In short, supply and demand.

Now, this is how it works for fungible tokens, like in the ERC-20 standard. If we're talking about non-fungible tokens, or NFTs, creators usually define a price for "minting" (creating a new token in a collection). This initial minting price may or may not influence secondary sales.