Yikes. You shouldn't have to pay $20 (plus shipping?) for a low speed gearmotor. If you are willing to wait a bit for shipping from Asia, as suggested above I'd think less than $5 is a better target price. Also, since the load is a light weight sign the need for a beefy metal gear mechanism is not so critical.
If your wallet is not flush with currency, how about pairing the following (from eBay):
I don't know what you're planning for a power source, but let's say you have Accessory AC 14-16V. Using the voltage converter shown above, you can use a screwdriver to adjust/set the output voltage to 3V DC, 4V DC, 5V DC, etc. to tune the DC gearmotor speed. Note that the gearmotor shown has a threaded shaft which can make things much easier - YMMV. Think carefully about how exactly you're going to mate your sign to the gearmotor.