Can't say I blame you for asking for sources. Sources are always a good thing. Contrary to popular belief, it's actually fairly easy to find good info on the JSF program. The DoD and Lockheed have been relatively transparent about the F-35's development compared to past fighter programs. I've done a lot of research on fighter development both out of interest and for my major (aerospace engineering student) and there is WAY more trustworthy info out there about the F-35 program than any other past program, plus cool pictures of prototype designs.
Anyways, my first point was based on simple logic. With one program developing 3 different aircraft that use many similar parts, you save a lot of time and money on design, development, and contractor costs. This was especially important for the JSF because a lot of the tech being put in the X-35 was downright experimental at the time, and a long development cycle was expected. With 3 separate programs, each aircraft would have their own contractor and would need unique parts, maintenance, and extra planning and oversight. Plus there would be less of each fighter being made, which can cause a sort-of feedback loop of unit costs getting higher, which causes less to be bought, which drives up unit costs further. With the JSF program all three aircraft help reduce the other's unit cost thanks to shared parts, and the cost savings can be easily observed when the F-35A costs less than many 4.5 gen fighters. While the cost savings were definitely not as radically efficient as the DoD hoped, the JSF program is definitely a cheaper solution than the development of 3 different fighters.
My second point is based on publicly available information about the JSF program. From the start, the JSF program was a multinational program. The UK, for example, will be paying around 10% of the total development costs. Many countries also stand to profit from the program. Canada, for example, is manufacturing a bunch of the electronics for the F-35.
My third point is also publicly available information, and is also proof of how little the media understands about the program. The F-35 program was presented in a different way than any previous aircraft development program. To be able to better plan for the long-term, the F-35's program cost wasn't presented as "$$$ so far", but as "how much $$$ this'll cost by the end of the program". Many news sources misinterpreted this (and still do) as the cost of the program so far, and the F-35 instantly became the "trillion-dollar fighter". Due to the sheer stupidity of the media, the DoD has gone back to using the old cost system for the B-21 Reaper.
I'm typing this on a phone at 4 in the morning, so I hope that made sense. When it comes to actual websites you can use for info, make sure you get none of your info from RT, War is Boring, or Pierre Sprey. All three have been caught deliberately spreading misinformation about the F-35 and other US military programs in the past. I recommend Aviation Week if you just want a layman's news source, but there are several fantastic articles written by F-35 pilots that describe what it's like to fly it in detail, such as this one by a Norwegian pilot (with an English translation): nettsteder.regjeringen.no/kampfly/2016/03/01/f-35-i-naerkamp-hva-har-jeg-laert-sa-langt-the-f-35-in-a-dogfight-what-have-i-learned-so-far/
Also, these four videos have a ton of fantastic info about the F-35 and debunks a ton of myths perpetuated by the media:
youtu.be/ZtZNBkKdO5U
youtu.be/LyHlp7tJrxY
youtu.be/31oJIo8EVwY
youtu.be/9s7-3EUXC_w
Also, this should back up my claim that "the media does this every time". Here's a NYT article from 1977 that's criticizing the F-16 for a lot of the same things the F-35's being criticized for now:
mobile.nytimes.com/1977/05/01/archives/f-16-fighter-program-is-hurt-by-delays-government-accounting-office.html