[enriquto]

good answer if the job is in a big and very expensive city:

"I like this job and would do it for free if I was rich. But I'm not: I'll need to live with this salary. The absolute minimum I require is food and shelter for my family (single dad of 3). Look at the rent of a comfortable 4 bedroom apartment within 10 minutes on foot to the lab, and multiply it by three. That number is the least that I'm able to take. But of course if you pay me more I'll be happier."