Here are a few of my favorite Pentax SLRs under $50. The MV-1 is cool because it's dirt cheap, has a corny red-green light instead of a real meter, and will have the shutter open as long as necessary for proper expose. No settings, just set the aperture. Very light and compact and takes excellent pictures with that 50mm f/1.4 lens I keep talking about. This is a fun camera for night pictures because you don't have to fiddle with anything but aperture and focus
Another favorite is the Pentax ME-Super which came out around the same time as the MV-1 but has much fuller features including a top shutter speed of 2000 (better than the MV-1 for daylight photography with a fast lens) and shutter and aperture priority. But still no aperture priority - really the only downside of this full featured, light, compact and nice looking camera.
The Pentax KM is like the K1000 with a self timer and an aperture preview (FINALLY). It's basically a Nikkormat/Nikomat FTn but in K mount (I'll get to the Nikkormats later). Also under $50 for this gem which is about half the price of the less-abled K1000.
A shoutout to the Pentax LX for being sexy. Too bad you won't find this one under $50