FEs and FGs (and Pentax ME/ME Super) are notorious for electronics failures (which may be manifesting as "various mechanical issues"). They were early movers on flexible-printed-circuit technology and it did NOT work out for them in the long term.

(as opposed to the mechanical rube-goldberg approach of previous models - a camera is basically a series of mousetraps each springing the next mousetrap in the series. Pushing the button triggers the mirror and the stopdown lever, the mirror triggers the shutter curtain, the curtain triggers the flash and the timer, the timer triggers the second curtain, the second curtain triggers the mirror down, etc etc. Electronics let you orchestrate that all a lot more simply, but early electronics weren't as reliable in the long run.)

Generally, people seem to go for all-mechanical cameras (usually only the light meter is electronic, or sometimes they don't have a light meter) because at least those are repairable when they break. For the Nikon line the FM1 and FM2 and FM2n are very popular, the F and F2, as well as the Nikkormats. For Pentax, the highest end camera is the LX, but they are known as somewhat flaky, the MX is very common and a very nice camera with an extremely large and bright viewfinder.

In general you may just have to resign yourself to buying something and sending it in to be serviced. You can seek out someone else who's already done it for you, and maybe you can get a better deal like that vs buying something cheap and sending it in yourself, but either way, they are mechanical devices and someone has to clean and lube them every 20 years or so.

Unfortunately a lot of the people who do that repair are extremely old at this point, a lot of them did repair work at $BRAND service centers in the 70s and 80s and branched out on their own when it closed down. But there is no equivalent pipeline training new repairmen and when they die, that's it, the knowledge dies with them. And a lot of them have warehoused a lot of parts, and unless someone else buys the whole lot then parts get a lot tougher when they die too. So if you want something, don't hesitate, buy it and get it serviced and hopefully you will only run into minor cleaning/seal replacement/etc type issues.

As usual, 90% Of Everything Is Crap. Nobody wants to shoot a Kodak Pony 135 (or even something like the Argus C3, which I'd argue is actually OK in comparison...). The stuff that's spiking in price is the premium, professional-tier gear that people actually want and that can produce high-quality results. There is also a preference for certain name-brands that have better service chains or parts availability as a whole - I have no idea who I'd send a Kowa or Tokina (or even Canon FD) camera to, but someone is always going to be able to service Nikon or Hasselblad. The ability to put modern glass like Sigma or Samyang onto an older Pentax or Nikon body is really nice too, it gives you a full-frame body fairly cheaply, so "mount is still in use today" is a bonus IMO.

(note you don't need to buy a digital of the same brand... mirrorless cameras can use almost all the glass if you focus and stopdown manually, and this is actually more accurate because most digital cameras aren't designed for manual focusing, while mirrorless usually have some helper functionality like "focus peaking" on the Sony line...)

I personally think coatings are one under-appreciated characteristic. Pentax was a very early mover on multi-coatings with "SMC", Nikon had their own version that is alright, Hasselblad struck a deal where they adopted SMC as the T* coating in exchange for trading a license to the Distagon lens design (SMC Pentax K28/2 and SMC Pentax 67 55/4 late variant), Fuji had the "EBC" multi-coating. A lot of other 70s cameras either were single-coated or had inferior multi-coatings, and suffer from flare much more heavily. When Pentax's SMC patents expired in the early 90s, everybody else quietly copied SMC because it was just so far ahead of everyone else.

Sorry I don't have more specific information about pricing, but it's been years since I really looked, and I was kinda shocked by the prices I saw recently too. I don'tknow that I would have paid the prices these days either, some stuff is 4x or more what I paid 10-15 years ago. But maybe this helps explain some of the market psychology a bit.

I personally think the Pentax MX and Nikon FM/FM2/FM2n or Nikkormat FT2n are some of the best options all around, but I'm sure everyone else has realized that too. I mostly shoot medium format these days, and the Fuji GW690 and GSW690 are still fairly reasonable and provide top-tier optical quality.