What I tend to do is to think of the absolute lowest price that would make it seem worthwhile to me. Think of how detailed you'd be willing to go, how fast/easy/costly the medium(s) would be for you to get done, etc. And only offer what you're willing to do, like don't promise backgrounds or oversized pictures for some ridiculous amount you don't think anyone will go for because someone usually asks me anyway and then I'm stuck with a project I don't want to do.

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"Well that's just typical! Five minutes to go 'till the most important party of my life and half the house has been wrecked by a gigantic sandwich!"