How much does the airline fare for a particular route fluctuate?

Fare prices are not infinitely flexible, even though it may look that way when you are trying to buy a ticket. Each airline publishes a finite number of fixed fares on a given city pair in its public tariff. The tariff is then distributed to every travel agent and every online travel agent and every other major airline in the world through a system called a Global Distribution System or GDS.

Whether a particular fare is valid for purchase to cover a specific set of flights on a particular day under particular conditions is a separate matter. But the point is, that every fare price point has been arranged and is published in advance of you searching for it.

Access to the global distribution system (or GDS) is not a free service, but you can purchase a subscription. I pay for a subscription to a service called ExpertFlyer.com, which re-packages GDS information onto an web service. KVS is a competitor which has a Windows application.

Here's the first twenty fares I found, in ascending order order by price.

Fare Basis  Airline  Booking  Trip Type   Fare           Cabin  Effective  Expiration  Min/Max  Advanced
                     Class                                      Date       Date        Stay     Purchase Req
NAK21AWN    UA       N        One-Way     26.00 (USD)    E                 11/11/15             21
OA21UNL3    AA       O        One-Way     26.00 (USD)    E                 11/11/15             21
OA21UNL3    US       O        One-Way     26.00 (USD)    E                 11/11/15             21
PL2ABON     B6       P        One-Way     26.00 (USD)    E                 11/11/15             21
PL2ABON5    B6       P        One-Way     26.00 (USD)    E                 11/11/15             21
UA21NR      NK       U        One-Way     26.00 (USD)    E                                      21
NAK14AWN    UA       N        One-Way     34.00 (USD)    E                 11/11/15             14
OA14UNL3    AA       O        One-Way     34.00 (USD)    E                 11/11/15             14
OA14UNL3    US       O        One-Way     34.00 (USD)    E                 11/11/15             14
PL4ABON     B6       P        One-Way     34.00 (USD)    E                 11/11/15             14
PL4ABON5    B6       P        One-Way     34.00 (USD)    E                 11/11/15             14
UA14NR      NK       U        One-Way     34.00 (USD)    E                                      14
PL2ABON6    B6       P        One-Way     41.00 (USD)    E                 11/11/15             21
NAG07AWN    UA       N        One-Way     44.00 (USD)    E                 11/11/15             07
OA07XNL3    AA       O        One-Way     44.00 (USD)    E                 11/11/15             07
OA07XNL3    US       O        One-Way     44.00 (USD)    E                 11/11/15             07
PL7ABSN     B6       P        One-Way     44.00 (USD)    E                 11/11/15             07
PL7ABSN5    B6       P        One-Way     44.00 (USD)    E                 11/11/15             07
RA21NR      NK       R        One-Way     44.00 (USD)    E                                      21
UA7NR       NK       U        One-Way     44.00 (USD)    E                                      07

However, the price you see here does not include government taxes, airport fees, or carrier insurance or fuel surcharges. Nonetheless those additional factors tend to be fairly flat within the same cabin on the same route.

Sorting by price descending instead, we have the following fares on the same city pair (limiting myself to the top ten in coach).

Fare Basis  Airline  Booking  Trip Type   Fare           Cabin  Effective  Expiration  Min/Max  Advanced
                     Class                                      Date       Date        Stay     Purchase Req
Y           UA       Y        One-Way     5976.00 (USD)  E                                      
Y           US       Y        One-Way     5951.00 (USD)  E                                      
Y           AA       Y        One-Way     5921.00 (USD)  E                                      
Y           B6       Y        One-Way     5918.00 (USD)  E                             -- / ||  
YUA         UA       Y        One-Way     1901.00 (USD)  E      03/04/16                        
B6DMN/WAUP  US       B        One-Way     1549.00 (USD)  E      12/11/15                        
B6DM/WAUP   US       B        One-Way     1549.00 (USD)  E                                      
YAA00UPY    UA                One-Way     1481.00 (USD)  E                                      
YUA         UA       Y        One-Way     1401.00 (USD)  E                                      
YUANR       UA       Y        One-Way     1401.00 (USD)  E      03/04/16                        

Some caveats.

1. In principle the carrier can file different fares on the same city pair that vary depending on the location of the sales office. For instance, a travel agent based in Yamagata might see a different list of fares compared to a travel agent based in London or Dallas. However, that doesn't tend to happen on US domestic fares, so I will ignore that complication. It does happen for travel inside South America though.

2. Airlines may also set up private or corporate tariffs for their preferred customers such as major companies or huge travel agents to whom they are willing to offer a discount in exchange for some guaranteed business. Obviously, this is commercially sensitive, so the information on these fares is not available to me.

3. Some airlines, particularly the newer, low cost airlines, do not use the normal GDSes to distribute their fares. This is cheaper for them, because GDSes are expensive, but it comes at a cost in that the airline must arrange a different mechanism for travel agents or websites to book onto their flights. In that case it is difficult to see how to measure the range they might move through.

4. The fare tariffs can be updated, they are not permanent. The GDS can typically handle a tariff refresh once per hour, but most fares will stay filed with only minor tweaks to the price for many months or even years. Sale fares do get added, but they don't replace the old fares (no one would buy the more expensive fare while the cheaper fare is available, but in principle you could).

5. The airlines have control over to whom their fares are distributed. Delta (DL) has decided it does not want its customers to be able to inspect its fares outside of delta.com, and has denied ExpertFlyer.com access to the Delta fare tariff. Therefore the list given is not exhaustive, but a travel agent could see the DL fares.

6. Multiple fares can be combined on one ticket (the ticket price is then the sum of each fare plus the appropriate taxes, fees and surcharges). For instance you could fly DFW-ORD-BOS. To cover those two flights, you could either buy a DFWBOS fare that covers the two flights, or you could buy a DFWORD fare and an ORDBOS fare, provided the two fares were mutually compatible [which as a travel agent, you must check based on the rules in each fare]. Therefore there is a further layer of complexity here that depends on your specific route, which makes the pricing problem substantially more complicated. More concretely, if you decided to route via Moscow, then obviously the fare would go up a lot. So here I am assuming a "reasonable" route (basically, no more than two hops, inside the United States).

7. The very top end fares are almost never going to be sold to customers; they are just for accounting purposes, a bit like the "rack rate" you see at a hotel on the back of the door. They exist so the airline can calculate certain discounts against a fixed maximum price, or more pessimistically, to rip you off when you seek a refund for a downgrade or to calculate the refund when you are changing your ticket ("your coach flight was worth 6000 USD so you don't get a refund on your 800 USD first class flight even though we downgraded you").