You can find some Springfield XD's in the $400s. I wouldn't be afraid to look at some stores for used guns. Just saw a used M&P 40 full sized for $419.
9mm would definitely suffice for a more comfortable carry caliber. As far as which gun to buy, the S&W Shield is a good choice a around $400. I recently bought an LCRx in .38 +p for about $440 out the door and so far i absolutely love it. If you are concerned at all about weight then either of these would be great.
You could grab a Canik at that price, and still have plenty left over for ammo. You might also look at a Taurus revolver (same thing applies), or I just picked up a SCCY XPC-2 9mm for only $239 plus tax. You can find some really good reviews on any of those options, which still leave you money for a box of ammo or two and a holster.