Many of the posters here have chimed in with a variety of CENTERFIRE pistols, but I will be odd-man-out here: I recommend a .22 RIMFIRE pistol. Here is why:
1. The $100 price tag for a decent centerfire is tougher to reach than with a rimfire. A used Ruger Mark I, Mark II, etc. "might" be able to be had at this level, if you shop carefully.
2. Small (read that cheap) centerfire pistols tend to have LOTS of recoil, due to the relationship between cartridge power and pistol weight. That makes lots of shooting unpleasant, unless you are immune to recoil. Most newbies are NOT immune to recoil.
3. If the buyer is limiting themselves to $100 investment in a pistol, I assume they are on a tight budget. That means they will be on a tight budget for AMMO once they acquire said pistol. A given amount of money will go LOTS further on .22 rimfire ammo than on ANY centerfire ammo. More ammo means more practice. More practice means more FUN, skill acquired, etc.
4. Many of the pistols mentioned (CZ-52, Makarov, etc.) are notorious for being difficult to use easily. (I carry a Mak for CCW, so I know from where I speak.) These pistol tend to slap your hand pretty good, or generate some pretty ferocious muzzle blast with cheap ball ammo. I consider them to be pistols best used by those who are familiar with centerfire pistols, and are accustomed to blast/recoil/etc. Asking a newbie to use one of these as their first gun is asking for them to become frustrated or scared of the pistol. That's not a good outcome.
It's pretty hard to start out investing in shooting handguns with only a $100 investment. Trying to do it with a centerfire gun is just making it that much harder for the reasons above.
