The Retina IIa rarely sells for less than $50, especially if it's been overhauled.
In the $50 price range:
-- The Olympus 35 RC is a nice little camera that has autoexposure, as well as manual exposure.
-- A couple of goofy little cameras are the Super Baldina and the Super Baldamatic.
-- Agfa has the Super Solinette.
-- Konica has the Konica C35, a rangefinder with program-type autoexposure but no manual exposure controls. The Konica S and S2 get rave reviews from those who own them. And before that, there is the Konica I and Konica II, which should be in your price range. The Konica III sells for significantly more.
-- The Voigtlander CLR and Vitomatic IIb can be found for about $50.
This is really the tip of the iceberg. There are probably hundreds of cameras that might fit your budget.