Well your math isn't wrong unless you consider I've been running Di2 for over 2 years and it has functioned seamlessly including the internal battery. In that time I've gone through at least 3 chains so that would bring the price comparison closer into line. Since I am still on the original battery so who knows how many more chains would come into the equation.
Even at the price if I had to replace the battery I would just consider it an acceptable price of maintaining the system. I like it that much. Crisp shifts and almost maintenance free. I don't miss replacing the cables but that's another issue.