Are there some countries that the Makarov is more desirable from? How does that affect price?
East German and Russian pistols have a higher collector value, and from what I gather they're a little better built (aesthetics wise) than the Bulgarian variants. They all go boom, though.
Gunbroker shows them from ~$100-$600.
A Bulgarian Makarov (cheap as you're going to find) should run you about $150 at any gunshow.
I assume that 9x18 is different from standard US 9mm parabellum?
You are correct, sir. 9mm Parabellum is 9x19mm.
There's really no "surplus" ammo that I know of, but the commercial stuff coming out of Russia (Wolf and Silver Bear) aren't horribly priced. I snagged a case of 1,000 rounds when I got my Makarov for $80 out the door if I remember right. It is a military caliber, after all. They tend to be fairly affordable, and unlike the rifle rounds, it doesn't look like there's any shortages right now.
(GigaBuist goes and fiddles around with the web for a minute.)
Okay, scratch that. Ammoman.com has a case of Wolf 9x18 listed as $179 shipped to your door. I guess the price of THAT went up too. Crikey!
A word of "caution": The PA-63 is sometimes called a "Polish Makarov" when it's not a Makarov. It just shoots the same caliber. Still not a bad pistol, but it's not a Makarov.
With that said, if your budget is $300 then you can get a Bulgarian Makarov AND a CZ-52, which realascout mentioned.
The CZ-52 is perhaps a bit less useful (fragile firing pin and a big 'ole hunk of metal) but it's an interesting piece and can be had for $130-$150 at gun shows around here. It's chambered in 7.62x25mm* and uses a delayed roller-locking blowback mechanism like you'd see in a CETME or H&K G3 rifle. Odd little critter.
*: 7.62 Tokarov and 7.62x25mm, I gather, are different beasts but dimensionally identical and I'm not sure if the CZ-52 can actually handle the "full house" 7.62x25mm rounds intended for sub-guns.