AMC Gold tickets carry a value up to $12. So for shows like 3D, IMAX, etc., you pay the difference. That is how one can calculate how much extra they will be charged when they use this.
Hope this helps.
I bought gold tickets for $4 a while back from another deal, and when i saw an IMAX matinee, i had to pay $5 surcharge for IMAX, even though i think the total was $12 or so.