Look at this METAR:
KLOU 151553Z 21005KT 10SM CLR 13/M01 A3044 RMK AO2 SLP309 T01281011 $
What does the dollar sign mean?
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.
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!