Translators’ productiveness may range depending on the text. For example, you can translate 500 words of an average text an hour but get stuck with the same 500 words of technical text for good 3 hours. You can, of course, define the rates depending on the text specialization. Or, as Corinne McKay describes in her article “How To Set Your Translation Rates By Using Objective Information”, you can set your rates per hour.
I hope you will be paid well no matter what you charge for.