But you'd also have to include the cost of the B-21, crew, maintenance, etc. - it's not only the $25k vs millions for a missile.
The B-21 is a fixed cost that is ready to go: just 'add water' (munitions) and you can have a higher pace of operations as long as you can feed the machine (B-21).
Still, strategic bombers add capability that drones/missiles don't. In particular, eyes on the target still haven't been deprecated by remote optics in drones and missiles can't loiter.