I don't think this is that complicated. There is a growing market for buyers, especially young ones out of college, who want a premium car but don't necessarily have the salary to shell out a premium price. The base 320 allows buyers, of any age, to get in a brand new BMW at a very reasonable cost. Mercedes is coming out with the CLA for the same reason; "sporty" sedan for $30k.
2015 BMW 428i Luxury Line Convertible - Imperial Blue/Saddle Brown
2014 BMW i3-REX - Andesite Silver/Giga Cassia Leather
2014 BMW 650i Coupe - Alpine White/Black Nappa loaded
2013 Maserati GranTurismo Sport- Nero Carbonio/Cuoio