Pistol Calibre Carbines
In your original question, you asked about rifles so my answer was mostly in terms of rifle calibres (excepting the Marlin 1894C).
I have a Marlin Camp Carbine in 9mm (the "Camp 9"), and it is quite simply the sweetest shooting rifle I have for work inside of 100 yards. It has what I call "ballistic intuition" and knows what I want to hit without my having to do any real aiming work.
I would certainly love to have their .45 ACP offering (the Camp .45), as I would imagine it's very much the same thing only larger.
If you ever get the opportunity to buy one of these, I say jump on it. They're a bit scarce, but a truly sweet gun.
There are a number of pistol calibre carbines suitable for home defense. Some of them are even within your stated price range.
Kel-tec makes the Sub-2000 series (already mentioned), which folds down into a briefcase-sized package, but which handles well.
Hi-Point makes the 995 carbine which, as already mentioned, fits in your wallet very nicely. One of the guys I work with bought one (under $200) and changed out the stock for the ATI "faux CX4" stock (under $80) and stuck a cheap red dot on it. He was hitting anything he wanted out to 150 yards. He said it "made it completely unfair" and "unsporting" 'cuz he could nail anything in a two-inch circle well past 100 yards. Ugly gun. Went bang every single time -- never had a malfunction.
Ruger makes a couple of PCxx pistol calibre carbines in 9mm and .40 S&W, but I believe they're over your price limit. They also do a .44 mag carbine (the 96/44 I believe), and that's a lever gun.
Beretta makes the CX4 carbine series -- but at $650+ you're well over the $500 line.
Now, back to the Marlins . . .
I don't actually think of the 1894 series (in either .357 mag or .44 mag) as pistol calibre carbines, even though they shoot ammo clearly intended for revolvers. The thing is, these same rounds, when fired from the longer barrel, pick up quite a bit of velocity, which turns them into serious rifle-quality performers.
(By the way, recent testing has shown that the .223 cartridge is quite suitable for home defense using hollow points or other frangible rounds.)