Nobody seems to have mentioned the SIG 2022. It comes in under the $500 mark, and - according to everything I've heard about them - are quite good.
I also agree with the Beretta 92 - if you like the grip.
Glock 19 - also if you like the grip
I'm a big fan of the PPQ, but it's a little over $500.
Remember you're going to want to spend a minimum of a hundred or so on ammo just to get used to a new gun.
9mm is going to cost less, and is going to be available in a wider variety of good range/defense packages than .38spl. A snub nose is not particularly fun or easy to shoot.
si vis pacem para bellum