What is a common percentage, if any, for an idea for something that someone else draws and produces?

Copyright protects the expression of the idea, not the idea itself. So unless your friend gave you very specific details about the design or you had an agreement to share the royalties with or turn over some or all of the copyright to your friend before you finished the design, he or she doesn't have much claim to your work. However, if you couldn't have come up with the design at all without your friend's inspiration, then he or she probably deserves some of the income from the work. It's up to the two of you to work out an arrangement.

This is a good question. Generally, "ideas" are not protected and that is why, before we divulge our ideas, we have the other party sign a Nondisclosure Agreement or a contract to act as partners in the deal. However, in your discussions or emails with your friend, you may have formed an oral contract. That depends on what was said and what was the plan or agreement, if any. Copyright protects creative, original designs once they are set into tangible form. That means you can register copyright on your drawing, but your friend could not register copyright on the idea for a drawing. However, as I just explained, you may have entered into an oral contract or partnership agreement to create the drawing with the goal of profiting from it. IF that is the case, you should be honest about this. If that is the case, you two may want to share the copyright on the drawing and split the part of sales that are attributable to the design. For examples, on tshirts, after you pay for printing and marketing and sales and shipping and all other expenses, you will have a profit. If 50% of that profit can be attributed to the design element, then your friend would get 50% of that. Printed novelty Tshirts can sell from $6 on up to about $50. You may want to form a contract with your friend to give them $1 to $3 per shirt that is sold, or some such thing. However, if there was no agreement of any sort, you may not owe your friend anything. But if you want to keep the friendship, and possibly even keep on collaborating on designs, then you should be generous, but get the deal in writing. Money is not everything. Friendship and sharing are important.