.22LR, .22, .22 short, and .22 magnum.
So, any of the .22LR or the PMR-30 Kel-Tec .22 Magnum pistol.
There's a significant jump per round when you go up to .25, .32, .38, .380, 9MM etc.
.380 is probably the smallest/cheapest round that is still effective for defensive purposes. (.38 is way bigger).
A lot of people go with 9 MM because it's all over the place and pretty effective. Rounds bigger than 9 MM will have yet another price increase.
It sort of depends on what you are doing though, you can't beat the price for a nice .22LR pistol and a couple bricks of ammo for plinking/target.