I had my first taste of negotiations with a gas station audit in a kinda away or in between civilizatiion setting, but just off an interstate. The MSC called me and asked for a price: I responded, "$20". I knew I could pick up another MS-Audit at another station for a total of $15 across the highway. The MSC responded with $20, not as the total but as the bonus. I was happy. $65 to drive 20 miles round trip just off the interstate on a bright comfortable sunny day. Two GS, 2 hours, 1 hour entry...done.
I suggest being vague the first time...throw out a figure. Like if I think I could accept $20 total, I just say $20. If they respond that's too high for a bonus, then ask them to counter offer. If all else fails and $20 total is acceptable, you have a fall back price.
After all two things: there is the price of what you are offering and the price someone is willing to pay. That is what a thing or service is worth. Number 2: always be closing. ABC!.
Do not read so much, look about you and think of what you see there.
Richard Feynman-- letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230