A potentially very expensive venture since you must get a gearhead that mates with the motor you have. And higher reducton ratios means a lot more money. I have found you can only get gearheads that mate with motors if you go straight to the motor manufacturer and get the motor AND the gearhead they made for it. Sometimes they refer you to another company that makes gearheads that goes with their motors, but it's pretty hard to match them up and make sure they fit unless you ask lots of questions or get them to send you lots of datasheets.
Either way, an incredibly expensive venture- not $20, not even $300.
EDIT: I notice you were looking for the whole shebang, and not just the gearhead, but the explanation above pretty much covers that case too. The cheapest gearmotors tend to be "chinese" or unnamed brands, but it's hard to find a large selection of those motors in any one place so you can choose the one with the specs you want. Some robot stores seem to have already done this or gotten deals or whatever, so check them out. The two that I have (but I don't like Lynxmotion for the sole reason they refuse to ship me with a non-UPS method, so I don't buy from them anymore).
Planetary gearheads: **broken link removed**
Spur gearheads: **broken link removed**
Your best bet is probably the planetary page since they max out 13RPM motors and the reductions tend to be slower speed higher torque than the spur gear motors. They are also all metal- some of the spur gearheads are part-plastic (pretty obvious when you look at the pictures). I also see that they recently added complete speed-torque-current graphs for some of the motors which is so much better than just listing the stall torque what they used to do for all the motors. So for those motors, you can check to see if they will do exactly what you want them to do.
If you need more power look at:
**broken link removed**
They are 24V motors, but you can run them at 12V. I never used the IG32 series, but for the IG42, make sure you compare the torque gained from the increase in gear reduction, because efficiency really starts to kick in. Once you go lower than 122 RPM, the amount of torque you gain for the decrease in speed ABSOLUTELY PLUMMETS. So I don't recommend you choose lower than 122RPM for the IG42 motors. CHeck this out:
Motor 1: 10kg-cm, 252
Motor 2:18kg-cm, 122 RPM
Motor 3: 20kg-cm, 75 RPM
So you can see that from motor 1-2, the amount torque you gain for the amount of speed you lose is "almost" proportional to each other, but from motors 2-3, you gain only 10% more torque for a 50% decrease in speed.
However, they are $30-$40 rather with the occasional low reduction gearmotor being $20. You wanted high reduction for $20, but you won't find much of a way around that.
One other alternative is to keep searching surplus stores for long enough- they are bound to turn up...eventually...but might not be what you are looking for.