So far as the pen itself - They've got some injection-molded plastic, where the mold costs are amortized across the production lifetime. A needle + syringe, which are probably subcontracted from Becton Dickinson. A spring. A printed label. And the medication itself (probably the #2 cost).
The assembly of the above all had to conform to the FDA's Good Manufacturing Practices rules. Every part & ingredient had to have trace-ability to their source. With documentation thereof.
When we got our device approved (in a looser classification than this) we had to get UL approval ($50k per test, per device, per test-run), and we had 3 people on staff whose sole purpose was navigating the FDA paperwork and their approval process. I never heard what that cost was, but it was substantial. And then we had an IP attorney, and corporate defense attorneys to respond to the inevitable lawsuits.
I worked at a summer camp where we would carry 1 or 2 pens plus a bunch of ampules if we were in a wilderness situation where we couldn't get to emergency care and needed more than one dose.
The marketing of autoinjectors has some legitimate public health value, but it's also a clear business strategy to get new drug prices for an ancient drug