Truly "affordable" combat aircraft projects don't look the way the JSF program did and does; there were actually affordable aircraft designs with which "affordable" wasn't just a marketing lie and eventually a running gag.
Some affordable combat aircraft have proven to be quite successful even if facing the high end combat aircraft of their own or even a later generation. How did they pull this off, was there any still valid trick?
For this text, I will consider the following example combat aircraft types as successful and "affordable":*
Saab Draken (Mach 2 interceptor, SWE)
Mirage V (Mach 2 ground attack, FRA)
A-4 Skyhawk (subsonic light fighter bomber, USA)
Folland Gnat (subsonic light fighter, UK)
MiG-21 (short-range Mach 2 interceptor, USSR)
SEPECAT Jaguar (supersonic ground attack, UK/France)
AMX (subsonic ground attack, ITA/BRA)
F-5 Freedom Fighter/Tiger II (supersonic light fighter bomber, US)
|F-5E in almost clean (dog fight) configuration|
(a) Speed: Slowness is not a necessity. Plenty supersonic entries with afterburning turbojets (Jaguar: afterburning turbofans).
(b) Engines: Single engine is not a necessity (Jaguar and F-5 had two)
(c) Avionics: No really powerful radar in the list, but some small radars are in the list.
(d) Agility: Most aircraft in the list were known for fine agility, albeit the subsonic ones have poor thrust/mass and Draken/Mirage had good agility primarily at high altitude.
(e) Undercarriage: Mixed; some are equipped for grass airfields, others not. The Skyhawk undercarriage was even dangerously high.
(f) Crew: Only single seaters, albeit with two-seat trainer versions available.
(g) All-round view: Only AMX and early MiG-21 approached 360° field of view. Most had only about 320°; this allows for more usable internal volume behind the cockpit (avionics black boxes, fuel).
(h) Weight class: Some were distinctly lightweight in their generation (Skyhawk, AMX, F-5), the rest was at least clearly no "heavy" aircraft in their generation (compare Mirage and Phantom II or Thunderchief, for example).
(i) Leap-ahead technologies: Mirage V was a downgraded Mirage III, which was among the first of the Mach 2 generation and one of few pure delta fighters. Draken and MiG-21 were also 1st Mach 2 generation aircraft. Draken pioneered the double delta wing.
(j) Timing: Most examples are from the 50's or 60's. The 1970's Jaguar was "affordable" only in context of its era. The even later AMX was affordable, but not very successful; it was apparently too similar to the performance of armed jet trainers and the time-honoured A-4.
(k) Jointness: None of these was developed as a joint air force and navy aircraft, albeit there was a naval Jaguar version (not produced due to French egoism).
(l) Multinationality: Jaguar and AMX were bilateral development programs.
(m) Air combat armament: None had medium-range (Sparrow class) air-air-missiles
I didn't really hope to discover the secret of how to develop and produce a modern combat aircraft on the cheap, of course. The quick look at the historical examples shows there's not one obvious formula for success.
Not being a "heavy" aircraft is a no-brainer.
Limited avionics suites save bucks (best examples Mirage V, Jaguar), that's also a no-brainer.
Two engines or supersonic ambitions are no K.O. criterion - or weren't.
Maybe the way to an affordable aircraft is
(1) use existing engine(s)**, don't strive for thrust/mass overmatch***
(2) use a small to normal size airframe
(3) use few or normal performance combat avionics
(4) two-seat versions primarily for training, not as main version
(5) good agility within the thrust/mass limitation
The rise of medium range air combat missiles and the growing importance of combat avionics for survival and targeting pose challenges for this 'recipe', though. Furthermore, even aircraft such as Jaguar and AMX were from their start badly challenged in their main roles by the rise of ZSU-23-4 self-propelled anti-air guns.
*: This list skips trainer-based aircraft and specialised ground attack aircraft because I suppose that truly affordable combat aircraft are an attractive option for small air forces, but specialised ground attack aircraft without a good deal of air2air capability are only good for civil wars. F-16 wasn't included because of the more low-cost F-5.
**: Jaguar didn't. Jaguar was kept in the list mostly for having a modest approach towards avionics.
***: F-16 did, and was almost certainly less cost-efficient than the older F-5 throughout the 80's..