Are we assuming that the two players have perfect knowledge of each others' prices?
A and B know each others costs and values.
That is a yes.
If A has something that B values at that price, and that can't be gotten anywhere else, he will charge what the market will bear; and the market will bear 500k, because that's what the phrase "B values the access at 500k" means.
This is not the case. In this scenario there is no special privilege for the resource that happens to be the service over the resource that happens to be money - the 'seller' doesn't arbitrarily get to dominate.
no special privilege for the resource that happens to be the service
I don't understand why this should be the case. Presumably A has other sources of money, but B has no other sources of access; unless you are specifying otherwise, there is an obvious asymmetry. If the situation is intended to be symmetric, the example is a bad one; it is cross-grained to well-established intuition about how money works.
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.