What does a dollar sign mean in a METAR?
A maintenance indicator sign, $, is included when an ASOS/AWSS (Automated Surface Observing System / Automated Weather Sensor System) detects that maintenance is needed on the system.


Reference: http://www.faa.gov/documentLibrary/media/Order/JO%207900.5C.pdf

Or if you need an easy way to remember it, courtesy of the awesome Ralph J:


  It means that they need to spend some money to fix whatever is broke on the automated reporting station!