Beyond the initial price tag, the B2's are said to be extremely maintenance-intensive.
The B-21's should be MUCH easier to maintain. That and spare parts are where the real savings will be.
The B-2s had about a $35 billion dollar development cost associated with the development of an all aspect stealth platform. Each aircraft only cost about $600 million, but the cost of the development was only spread through 20 aircraft, that is where the $2 billion dollar price tag comes from. The whole program cost $44 billion, divided by 20 aircraft equals $2.1 billion. The B-21 will have similar development costs that will hopefully be spread through a large fleet, hopefully over 200 aircraft.