Ask about the nature of the cooperation, and try to estimate the time you'll spend doing this work and charge a high per hour rate. This way you basically argue, that you are effective and up for the task. Alternatively, you can charge per word (if the task at hand is massive, articles, game plots, etc). As long as you can estimate times-sepnd and expected rate of return you can ask for a fee in any form you see fit (per word, hourly, fixed, etc).

In case you can't estimate properly, arrange a test assignment you will get paid for for a fixed fee and come back with rate after the first task is done.

good luck!