As PHPnoobie said, each developer is different. I personally charge based on the job and require an additional 45% of the final price to keep the plugin exclusive to you.
It's in your best interest to just post your job, with as much detail as possible, and then let someone who is interested contact you. If you don't like their rates you can always choose to wait for a better deal (provided one exists).
I would hazard a guess that most rates, regardless of how they are calculated, are going to be pretty close depending on the type of work you want done.
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