Cheap or expensive is always relative.
The USAF has a force-structure goal of 1900 fighters in inventory, of which 1100 are primary-mission aircraft (that is, available to operational units). The divestment of the A-10 and other planned cuts would leave them 334 aircraft short, according to 2014 testimony.
http://www.armed-services.senate.gov...s_04-08-14.pdf
The question is whether and when the USAF will be able to buy back fighter squadrons that it cuts today.
Those cuts are very closely linked to the growing fleet of non-combat-ready F-35s, which are drawing heavily on money and maintenance people. So the costs of the program are already drawing down the numbers in the force.
Also, both the F-16 and A-10 were, in their day, explicitly planned as lower-cost than, say, the F-111 and F-15; so while they were not in absolute terms cheap (like a MiG-21) they were relatively so.
The Navy denominates its force structure in TacAir squadrons, with a goal of 40 squadrons to maintain 10 CV Wings with 44 strike fighters. (Growlers not included.) Without a Super H life extension the Navy needs to add >3 F-35 squadrons annually in the 2020s to hold at 40 squadrons, which is unaffordable at $150m APUC.
The result is that the math works like this: each year, the Navy needs to cut X number of F-35Cs from its nominal buy (20 aircraft) until it has enough money to SLEP enough F-18s fast enough to fill its squadrons. Fortunately the equation closes because you can SLEP several F-18s for the price of one F-35C.