When this happened to me, I used legitimate data that was in my favor. I was graduating college and used the published salaries of graduates from my department as published by my department.
You can find numbers from glassdoor or look at what government salaries are for your job type in that location.
Just know that if you provide an acceptable range, they will lock in on the bottom of the range.
I've had reasonable success answering this question by saying something along the lines of "I'd have to consider the full offer, salary, benefits, etc."
If pressed, have a number ready, that's somewhat higher than what you think is realistic. Unless you are so far out of the ballpark that you look ridiculous, a high initial salary demand is not going to knock you out of consideration if you're an otherwise good candidate.
Actually, you always want to provide your own numbers first. That's what they call `controlling the document' in the legal profession, and `anchoring' in Psychology.
It's more common advice to never name the first price.
I'm no expert negotiator but personally I think this depends on the situation. In more than one case I've underestimated my worth and received more than I would have though I could get away with asking for. Once they name their price, I know it's their lower bound, so I'm comfortable asking for even more.
I think negotiating a salary has different dynamics than negotiating a sale. For a sale, I would probably agree with the anchoring tactic.
You can find numbers from glassdoor or look at what government salaries are for your job type in that location.
Just know that if you provide an acceptable range, they will lock in on the bottom of the range.