Reliably expanding under what conditions?
There is a lot of research and development that goes into modern duty in defense of ammo. But let’s assume you’re premise is accurate.
Retail pricing for duty in self-defense ammo in 20 and 25 round boxes is expensive. However even buying in bulk at agency or government contract pricing duty ammo is noticeably more expensive than FMJ training ammo. The main reason for that is not the cost of the projectile but rather the level of quality control that goes into factory duty or self-defense ammunition. I see quite a bit of both duty and training ammo go down range and it is extremely rare to see a defective or non-firing duty round from speer or Hornady.
Conversely we see a much higher rate of defective rounds or rounds which failed to fire in both spear and Winchester training lines.
As with guns and gun parts you get the level of quality control and reject rate that you are willing to pay for.