Airlines like Delta use a fare class. For example, their class structure is

F P Y B M H Q K L U T

They offer tickets at each class and when the tickets for that class is used up it goes to the next group. So lets stay they open a flight and offer 10 "T" class tickets, once they are sold you purchase "U" etc. The lower down the list the less expensive they are. But the overall price of the ticket is still determined by a base fare plus all the taxes and surcharges and the base fare seems to be increasing these days as the airlines try to find the top of where people are still willing to buy (and if they don't buy they will eventually lower the base fare price to get the tickets sold).

I fly Delta quite a bit and found that learning the system is always helpful (including rules for IDB/VDB, upgrades, medallion structures, etc). Its gotten me out of quite a few travel mishaps over the years.