When it comes to my family's safety I am not concerned with $5 or $10 savings on a box of ammunition.
I believe what the previous posters are trying to tell you is that if the ammo cost less you will shoot the pistol more. More practice equates to better shot placement (hopefully), familiarity with the firearm all if which really takes the caliber out of play. A well placed small round is far superior to a large caliber miss.
If you consider the average SD engagement occurs at between 7 - 10 feet you will not find much difference in performance between calibers.
Purchasing should be based on what fits you best and you enjoy shooting. Most of you cheaper firearms are not as much fun to shoot simply because you have to fiddle with them to make them work. After all a very nice, expensive pistol stuck in a drawer somewhere is no match for a shooter comfortable/capable with his Bersa at the ready.
The pursuit of the firearm is actually one of the funnest parts of firearm ownership so enjoy it.