A friend started a company and wants to give up some % of sales if I help him find new customers in the US who'd sign a contract which results in revenue for him. What % of revenue from order could I ask for?

5/15 and if this was not your friend but a stranger you could even do 10/20. This would be for, say the first term of the contract, say 1 year. After that, for renewals the commission becomes say 1/3 of the first year.