Single Shot 20 ga?
Effective for the 1 shot that it has, but once you've shot it, you are up the creek. So you better be a good shot, and there better not be multiple attackers. Still, it is much better than a sharp stick for the 1 shot it has.
For just a bit more money, you should be able to find a used .38 Special/.357 Magnum revolver that will give you more shots. Avoid the "no-names", get a used S&W, Ruger or (newer) Taurus. Rossi might be an acceptable low-cost option, for that matter.
For about the same amount of money, you can get a HiPoint, for that matter. Yes, they are heavy and homely, yes a lot of people sneer at them, but the newer ones do tend to work and they'll give you more shots. Also, HiPoint is known for honoring their warranties. Consider their carbines as well as the handguns.
Likewise, you should consider a Makarov -- you can get those for less than $200, and they'll certainly get the job done. Ammo is cheap, too. If it is currently outside of your budget, I'd suggest saving up for it, they are worth that little extra cash. Same thing for the used revolver suggestion above.