it depends mostly on how good the design is. even if you are a well established professignal designer, but created a bad design, it would be unfair to charge what you normally would.
can you post a link or pictures? thats the easiest way to measure your design's worth. generally a professional can charge $50-100+ per hour depending on their talents.
however if you are just an amatuer who doesn't normally do this (which it is indicated you are), you cannot hope to charge anywhere close to that much.