If I were you, I'd not think about it as what price the image should be. When I take on a new commission, I think about it like this: how long will this take me and how much is my time worth? Simply multiply a reasonably hourly rate by how long you anticipate it taking.

If you're an amateur, $15 an hour might be reasonable. Graphic design professionals might charge hundreds of dollars for an hour's work (though the difference might not be as huge as it seems since they probably work much faster and have more time saving tricks in their toolbox). Anyway, when you do it this way you don't need to feel bad about whether they turn you down or not. If they aren't willing to pay what your time is actually worth, then that is okay.