The 9mm is just a very shooter friendly cartridge. Low recoil and good inherent accuracy are two advantages. Its a very popular round and ammo can be found anywhere. Like you said ammo is cheaper than most others except possibly the .22 long rifle. That is a very important consideration if you are going to shoot much. The 9mm isn't that week, lots of people carry them for self defense. They consider the low recoil and controllability to be more important than more power. You could always buy a .357 Magnum revolver and shoot .38 Special out of it when you want less recoil and cheaper ammo.