They're very similar; using the same barrels, locking setup, and mags. So their accuracy and reliability are essentially the same.
As you know the "triggers" (which really are the entire system) are what separates them.
The P250 is a smooth, light, Double action only trigger that appeals (almost entirely) to revolver fans or risk adverse law enforcement agencies that prefer double action.
The P320 is a full cocked striker with a short, light, crisp trigger pull which appeals to the broad market of civilians, military, and law enforcement.
As such, the P320 is more expensive because it has a massive market that is willing to pay more than for the P250. Part of the P250's biggest selling point is because it IS cheap.
It's been a while since I've torn them both down, but while both are pretty simple, the P320 does have more parts and some more complexity to it.
Think if it this way: The P320 isn't more expensive, the P250 is just cheaper.