Look in the mirror and start listing off salaries, starting with the one you would first expect and then start increasing it.
$100,000. $120,000. $150,000. $175,000. etc.
As soon as you can’t look at yourself without smiling and laughing, that’s where you stop.
That’s the top Quora answer to the question of picking your own salary in a negotiation. See the link for a more complete response from Jason Calacanis.
If you’ve had a hard time thinking of a price, use your gut. Often the seller is happy just to get a response. And you never know…