Most defensive ammo has specially designed bullet profiles that are designed to expand in a target and stop (i.e., not to exit and cause any collateral damage) These are a bit more expensive to manufacture than standard round nose.
Also, because defensive ammo MUST go bang every time, the manufacturing processes are usually a lot more monitored. You also get higher quality components in terms of powder, casing, etc.
Most defense ammo is also full charge, or holds more powder.
All these add up to higher manufacturing costs.
And then, they are given a special brand name that obviously has to cost another 50% or more.
The fact is, defensive ammo is usually the most consistant and reliable factory ammo you can buy. And you pay a premium for that.