First, there was nothing magical about 50mm on FF. The only reason it got called the classic/standard/normal etc. was because a standard 50mm lens is THE easiest lens to make and it's very easy and inexpensive to make one that is super sharp stopped down just a little. So it might be more rightly referred to as the simplest/cheapest focal length than the classical/standard/magical/etc. So they used the be the kit lens years ago.
Actually, there are a few "Magical" reason behind the 50mm standard lens. The diagonal of a FF is 43.3mm. In the old days, 45mm is the standard focal lenth for fixed lens camera. Also the angle of view of 45mm is also almost the same as the "clear view" angle of human eye. Leica was using 50mm as standard lens since day one. In the 30's comes the SLR (Exakta). Due to the frange focal length, that make it impossible to make 45mm focal length for the camera. So 55mm was forced to be the focal length for its standard lens. This focal length was adopted by almot all SLR maker until the 60's. As techology gets better, SLR maker finally was able to make 50mm as standard lens.
As for the price, 50mm is always the cheapest due to high volume, not due to the ease of design or manufacturing. All the design cost and manufacturing tooling are shared by millions of lenses. That makes it almost zero overhead. As for ease of design, for SLR it is easier to design a good 60mm lens than a 50mm lens due to the frange focal length. But 60mm is a little too narrow for general use.