This is generally accepted as a good camera to learn on. They are simple and reasonably robust with a decent standard lens. You will certainly find a good example in your price range, and still have something in hand for another lens.

Another worth considering is the Nikon FM2, you should be able to find one of these within your budget nowadays, like the Pentax it is basic, but has a greater range of shutter speeds with easier to use (in my opinion anyway) metering. There are plenty of lenses available for both of these cameras. The Nikon is probably more robust mechanically.