Do airlines assign a specific fare cost to each fare class. For example, if I wanted to fly SFO-ORD on United, would (made up scenarios) A or B or something else be correct.

A)
N fare is $90
G fare is $100
....
Y fare is $2,000
Lowest fare available can be controlled by availability of fare classes

B)
N fare is cheapest
G fare is 2nd cheapest
....
Y fare is most expensive
Lowest fare available is set and UA somehow decides which fare class that it will release into.

I was pricing out an SJC-PHL on UA and it was about $250 one-way, and I was shocked when I got to the payment screen and the flights were both in N because I had bought tickets on this route for half that price in N.