Tiered rates would ONLY work if it were tied to tiered license/permits for which you had to qualify.

Just some random ideas:

Planes under under 24 ounces & 30 mph - No test/qual required
Planes over 24 ounces or over 30 mph
planes over 60 ounces or over 60 mph
Planes over 100 ounces or over 100 mph
Planes over 160 0unces or over 150 mph
Above that.

No special rational here, just illustrating what I meant. To have an AMA sanctioned membership and insurance coverage you would need the appropriate membership and have passed the appropriate level of certification.

An un certified membership would be equal to the lowest level.

Using the scale here, anyone could join for whatever $$ and fly that class of plane and have full coverage and privledges. Now, if you are flying a 5 pound plane and hit something, don't bother to file an insureance claim cause you ain't covered!