This depends on what the person has in his or her possession and willing to risk to trade with. But it should be the minimal amount of money you know you can possibly risk in a game and still feel not bothered when lost to trading. 2% or 1% of a person’s income may vary for different persons on the total price it will amount to and also could vary on the amount they’ll be willing to lose in a trade, so the percentage of the actual income to risk depends on the person and the amount they’re willing to lose.