Alright, I got my monitors. They're really small to match a small room and a small budget. Is there a cheap subwoofer out there that will be able to do *something* for me? I'm thinking like, $50 and less?

David,
 
$50 or less? .... Probably not. You might check Craig's List or eBay for something used. You never now!
 
I bought a Tannoy PS110B some years ago that I really like. Gives me some bottom end under the 5" speakers.

i have never seen/heard a decent sub for less that about $300

Lets say I'm a noob. Let's say, I know little-to-nothing about what a GOOD system truly sounds like (because my ears aren't yet trained). How will I know when I come across a 'good' subwoofer?
I guess I need guidance because I bought a pair cheaper monitors/speakers and to me they sound great, but then I have no clear concept of what 'great' sound like.
On a note more in the vein of the topic, I have no idea how I'd gauge a 'good' subwoofer, and why there'd be no inexpensive, quality alternatives like these monitors!
 
Then again, I'm a noob 

Training your ears that "cheap" mons sound good is not a good idea.
Go to a big box store like GC and compare speakers. I use Adam A7x so that is my benchmark, personally.

i use a very inexpensive set of monitors,
the JBL 305 series...
 
they sell a matched sub with it...
i really don't need it in my room,
so i passed...
 
but i would suggest going with a matched set,
as expensive as you can afford.
 
don't try to mix/match non-matched equipment,
it's just going to waste your time.

 
I think it would be better to have two decent monitors (even cheap ones) with at least 6" speakers than to have two small monitors and a really cheap subwoofer.

For $50, don't bother.  You may be able to get a used home theater thump box, but it won't tell you anything useful about your music.  Pro Subwoofers can be rather expensive as they include powerful class D amps, heavy construction,  variable low pass filters and usually a 2 channel summing mixer. "Cheap" is under $500, JBL or Yamaha make decent ones as low as you should be expecting to spend.