Kukkiwon 1st Dan - $70.00
Kukkiwon 2nd Dan - $90.00
Kukkiwon 3rd Dan - $120.00
Kukkiwon 4th Dan - $150.00
Anything over and above these costs are at the discretion of your instructor. Only a Kukkiwon-certified 4th Dan and above can sign off on your application for a Kukkiwon certificate. So, is your instructor a 4th Dan, or does he need to have someone else sign your application? If so, it may be part of his $500 fee.
To answer your question, I paid $400 for my black belt test/certification 2 years ago.
Hope this helps and good luck on Saturday.