The cheapest to shoot of the what would be considered self defense cartridges is th 9mm. Next would be the 40, then the 45. Most likely all others will be more expensive than the 45. Unless you reload. I'm not going debate caliber wars. I shoot and carry both the 9mm and the 40. And I feel well protected. The best thing if a local range or friends has the different calibers to shoot to feel the recoil. For some they don't like the recoil of the 40. If you buy the bigger caliber like the 40 or the 357sig. You can get conversion barrels for the 9mm.
Good luck in your Glock search.
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22C Gen 2, 17 Gen 2, 26 Gen 4, 30 SF
40 Club number #164