A Franka Rolfix
They sometimes come up on the dollar table at the Photographica show. I picked one up and found the quality to be surprising good. It's not a Zeiss, but not bad at all. There are others in the same ballpark from the late 30's through the advent of coated lenses and common rangefinders. These were family cameras of the lower middle class (my family is a good example) and they were meant to be serviceable, not fancy. One of my own favorite "given away" cameras is a Kodak Monitor. A lot more expensive in its day, it has bells and whistles the Franka doesn't, but it's heavy, old fashioned and not immediately useable for most people (620 required). The Tourist or the venerable Six-20 can be found pretty cheaply at times as well. If you're looking for cheap, and you don't mind re-rolling film, 620 is the way to go.
I've been spending a lot of time processing donations to PHSNE in preparation for shows and auctions, and I can tell you that there are a lot of cameras out there that will cost less than $30 but are worth shooting. You just have to look around a bit and try your luck with more than one source. Ebay is not your best bet these days. You may have to loosen up a shutter or clean up a grungy case, but you get that with "cheap."