It depends - if everyone knew that B would provide a superior wall, in less time, for the same money, then they'd have a choice to make. Do I care about the build quality? Do I care about the time?
A better example is if A now comes back and says "yes, my wall's not as good, and I take longer, but now I'm only going to charge $4000", then you've got a bigger question - is it worth $1000 to you to get a higher quality wall that's done quicker? Or would you rather save the money and lose the time and build quality?
This more easily converts to a software example - do you pay a senior developer to develop your product quickly and well? Or do you save money by hiring a developer of lower skill, who will likely take longer and deliver a lower-quality product?
You're making a key assumption that a person is capable of evaluating the skill of a developer and the quality of the end-result.
What if you have no background in software? If the lower-skill developer convinces you his work is better (even if it's not) you're going to go with him.
"if everyone knew that B would provide a superior wall, in less time, for the same money, then they'd have a choice to make"
I'm more inclined to think that people would try to bargain hard with the Bricklayer B - "$500 in two hours is a good price for you, pal". After all the thought goes that even for that price B can erect a lot of walls in two weeks (which is A's single wall building time-span), and that's... only fair, right?
(in addition) Don't foul yourself that we live only under market-like laws of supply and demand, we're also social creatures that supposedly have to obey social norms. And the norms can be dictated.
A better example is if A now comes back and says "yes, my wall's not as good, and I take longer, but now I'm only going to charge $4000", then you've got a bigger question - is it worth $1000 to you to get a higher quality wall that's done quicker? Or would you rather save the money and lose the time and build quality?
This more easily converts to a software example - do you pay a senior developer to develop your product quickly and well? Or do you save money by hiring a developer of lower skill, who will likely take longer and deliver a lower-quality product?