That (insane) $18 charge will only occur when demand for parking is at a fevered pitch, such as it is during Fleet Week when the Navy's in town. Normal charges, however, will range from a low of $0.25 per hour up to $6.
Sensors in the parking meters will monitor parking space availability, and hourly pricing will then be dictated according to the perceived demand. The project is called SF Park, and it's currently funded to run for two years. Perhaps the name SF Gouge is more appropriate.
[Source: KTVU.com | Image: OrinRobertJohn/Flickr, CC 2.0]