Although these are not 50mm lenses, they offer about the same field of view as a 50mm one. I refer to the 25mm AF lenses produced for the micro 4/3 system, Panasonic and Olympus, more precisely the 25mm f:1.7 Panasonic Lumix lens.
This lens is the cheapest “normal” or “standard” auto focus prime lens for the micro 4/3 system. It competes with Panasonic’s own 20mm f:1.7, Panasonic Leica DG 25mm f:1.4, Olympus Zuiko 25mm f:1.8 and Olympus Zuiko Pro 25mm f:1.2 lens. Quite a big offer here in the price range of 250 USD and 1200 USD. As said, the Lumix 25/1.7 is the cheapest in this range while de Zuiko 1.2 the most expensive.
Here are a couple of reviews on the Lumix 25/1.7 lens: