I just found in California VC5036. The fee for a special motorized bicycle plate application is $15.00 Effective January 1st, 2004.
However VC1678 lets them increase it 50 cents a year if they feel like it.
That explains why it is $18.00 now......
The application is DMV form 230.
Here's a link to it:
http://www.dmv.ca.gov/forms/reg/reg230.pdf
Here's the link to all the California laws, ect:
California Vehicle Code Division 3, Chapter 1, Article 8.1 Table of Contents
or
Vehicle Code Index -M- Scroll down to motorized bicycle.
I hope all this helps clear any confusion.