I think assuming away the secondary variables in a negotiation problem is less interesting/useful than assuming away friction and air resistance in a physics problem, for tworeasons.
First, air resistance usually explains only a small fraction of the variance in outcomes -- my numbers won't be quite right, but the distribution of physics parameters across all back-of-the-envelope physics calculations will probably be something like: log(air pressure in atms) = 0 +- 0.2, log(density in g/ml) = 0 +- 0.3, and log(initial velocity in m/s) = 1 +- 1. If you vary the air pressure by one standard deviation, you get maybe a 3% change in the distance traveled. If you vary the initial velocity by one standard deviation, you get maybe a 200% change in the distance traveled. So it's relatively safe to leave out air resistance. By contrast, bargaining power, precommitment, BATNA, transactional costs, etc. often explain as much or more of the variance in the agreed-upon price as does the deal's utility to each person. It's not at all safe to leave those factors out -- if you do leave those factors out, it's unlikely that you'll get even 4 full bits of Bayesian evidence about the final price as compared to a prior that allows any price between $0 and $10 million.
Second, air resistance is much harder to measure than initial velocity, and, even if you figure out what the level of air resistance is, introducing that variable makes the math considerably harder -- you might have to upgrade from algebra to calculus. Each additional variable superlinearly increases the risk of calculation error, because physics reasons from explicit mathematical equations whose counterintuitive results should be trusted, assuming the math and the inputs are correct. By contrast, it's often just as hard if not harder to measure the utility a person gets from a deal as it is to measure, e.g., their BATNA. Adding new variables to the calculation doesn't necessarily add much to the difficulty of the calculation, because there is no unified, complex mathematical equation -- just Bayesian updating on what each piece of data has to say about the overall likelihood of any particular deal. In other words, if the deal is worth a lot of money to you, that suggests that you're relatively more likely to agree to a higher price; if your BATNA is quite good; that suggests that you're relatively less likely to agree to a higher price. Total up all the evidence pro and con, and you'll get a sense of where the price is most likely to be. There's no need to factor complex mathematical expressions; at worst, the complexity of the calculation increases at a linear rate as new variables are added.
This puzzled me. I'm pretty sure it's one of those unsolvable questions, but I'd want to know if it's not.
Two members of the species Homo Economus, A and B, live next to each other. A wants to buy an easement (a right to cross B's property, without which he cannot bring anything onto his lot) from B so that he can develop his property. B, under the law, has an absolute right to exclude A, meaning that nothing happens unless B agrees to it. The cost to B of granting this easement is $10 - it's over a fairly remote part of his land and he's not using it for anything else. A values the easement at $500,000, because he's got a sweet spot to build his dream house, if only he could construction equipment and whatnot to it. A and B know each others costs and values. They are "rational" and purely self-interested and bargaining costs zero. What's the outcome? I'm guessing it's "Between $5 and $500k," or "There is no deal unless one can credibly commit to being irrational." But I'm really not sure.
This could be asked as "In a bilateral monopoly situation where the seller's reservation price is $5 and the buyer's is $500,000, what is the predicted outcome?" But I figured the concrete example might make it more concrete.
Now that I've written this, I'm tempted to develop a "True price fallacy" and its implications for utilitarian measurement. But that's a separate matter entirely.