make a big number up and ask. be willing to go down if theyre budget doesnt allow it tho. if they brag avout "millions" of dowbloads then id ask around $300-$500 or so, but initially say $700 and go down
i dont do percentage deals. i just want cash up front, with custome written terms of service on the invoice. terms saying something like "ok youre only paying me $150 right now but if these shirts sell more than $5,000 in revenue, you owe me $150 more. if you make more than $10,000 you owe me $1000 more along with 5% revenue gathered from said product i helped be a part of"
just to be sure youre clear about what ur doing.
Great words of advice, thank you.