It is commonly understood that writing professionals get paid by word count. And most often it is so.
But still, it is not the only way of defining the charge for your work. And in some cases, word count is just not the right method to estimate the work you have done.
Writers should not aim to create a long text, but a good one. In that case, getting paid by word count may lead you to the longest novel you’ve ever written, but not the best one. So in her blog ”Should Writers Be Paid By Word Count?” Laura Spencer suggests setting the quote by word count range (for example, “around 3000 words” or “600 to 800 words”).
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.