There are a good number of times when it can cost one party little to satisfy a significant need of the other party. If each party can offer something of low value to the offeror and high value to the other party, this presents a wonderful opportunity for trading that will generate higher overall value in the deal. This type of trading, known as logrolling, can be a source of great satisfaction.
The risk analysis discussion above should already suggest that a good negotiator should not shy away from numbers. In deals there are often many moving parts, each with its potential economic value. It pays to try to price values, to calculate risks, to test principles and assumptions by working out their math.