It's a hard question to answer.
I've set $20/hr as my personal goal for writing. And I'm making it, but it averages out strangely. I might spend twenty hours on a short story that will only earn $60 because I want exposure to a new market - I'm essentially using the story as an advertisement, not as a tool for earning money. I compensate for that by making more than $20 an hour for my novels, at least so far, but maybe I wouldn't if I also counted all the time I spend networking and writing promo work and whatever else.
Also, I'm essentially just playing a game, because I love writing and would do it for free. I also have a stable, well-paid job, so I don't NEED the extra money from writing.
If you're trying to make a living at something, you need to take things a bit more seriously. But I'd ask yourself a few questions:
What can you afford?
Are you taking time away from other projects to work on this one?
Roughly how much would you earn on the other projects?
How bad do you want this job? (ie. is it just another job, or would
it give you valuable exposure, a foot in the door somewhere, etc.)
What's the market like (I know, this one is tough)? What can the
company afford to pay you?
Those last ones are probably the ones you're having trouble with, right?
You may want to put the question back to the people who contacted you. Respond with a writing sample, as they've requested, but say "rates negotiable". Let them come to you with a number. It might work!