Patricia:
You do NOT need to know what an appropriate fee would be.
You have OTHER OPTIONS:
Post this as an HOURLY job.
Then you don't need to specify a dollar amount.
Alternatively, specify that the fee is $500. And at the top of your job description write:
"I do not know the appropriate dollar amount to specify. $500 is a placeholder amount. Please read the job description and bid an appropriate amount. Any proposal that specifies $500 will be ignored.