1st class D train tickets on this route about RMB 120.
Why would you even be considering a flight on this route? It takes longer to get through the airport rigamarole and then endure the inevitable flight delays. Trains are like buses, should be able to buy walk-up for next out. I'd take my chances doing the ground transport switch by taxi to airport in Ningbo. Sounds like you don't have business/stopover in Ningbo. I don't know answer to your mileage question or the luggage rail-to-plane transfer question, don't know anyone who's tried this or even inquired of MU.

Which brings up another question? Are you going all the way to Ningbo just to catch a plane? Why not change and fly to wherever from Shanghai? Your profile says Shanghai so I'm curious why you don't already know all this, or found answers locally.