Between $300 and $500 you've got a ton of options.
For new guns, the vast majority of polymer 9mm's on the market will come in at under $500. Glock just barely breaks that (though used they're well inside that range - I paid $265 for my Glock 17 used), and Walther might be a bit higher, but you can get new guns from S&W, Ruger, Kahr, Keltec, FN, CZ, Caracal, Springfield, and several other manufacturers for under $500.
Just personally, I think a good "Dual purpose" gun for around the house, concealment, and some range use is the Ruger SR9c. Runs right around $400 and not only has a compact grip 10-round mag for concealment, but they also include a full-sized 17-round mag with a grip sleeve that you can use for range use, home defense, etc, that gives the gun a full-sized grip. Of course if you want you can get the same thing with a Glock 26 and a separately purchased G17 magazine + sleeve as well if your preference runs that way.